Why are amaretti mixed as they are?
Most recipes for amaretti call for the sugar to be combined with the ground almonds and then added to beaten egg whites. When making meringue, on the other hand, the sugar is beaten into partially-beaten egg whites. What is the reason for this difference?



When making meringue you want clean equipment and to avoid oil or grease in order to be able to whip the eggs and obtain the correct foam with stiff peaks. Sugar is added in spoonfuls between the soft and stiff peak stage. See #2 for information about adding almonds.

Sources:

Felicity Cloake: "How to make the perfect meringue":


  "One of the golden rules of meringue making is that all of your equipment must be scrupulously clean, without a speck of grease, or it will be much more difficult (although not impossible, as is often claimed, according to the food chemist, Hervé This) to produce the desired foam with your ingredients.
  
  ...
  
  Most recipes call for the sugar only after the whites have been whipped to soft peaks – add it too early and you can kiss goodbye to a good strong foam.
  
  ...
  
  You need to put them in at 60C, 70C, overnight." You don't cook meringues so much as dry them out, apparently; evaporating the water to leave only the rigid structure of the egg and sugar mix, and the air bubbles in between.".

If you grind almonds by themselves they will become oily, so some or all of the sugar called for in the recipe should be added to the almonds. 

Sources:

David Lebovitz: "What is Almond Flour?":


  "... To make your own almond flour, I highly recommend that you grind the almonds with some of the flour or sugar called for in the recipe (use the weight equivalent for better accuracy); nuts ground on their own can heat up during the pulverizing and get oily. Flour or sugar helps prevent that. Generally about 2 tablespoons of flour or sugar is the minimum you should use per 1 cup (120g) of nuts. Avoid processing more nuts than that at a time. If you need more, work in batches.)
  
  To make your own almond flour, put the untoasted nuts in a blender or food processor and pulse them with the dry ingredients until the almonds are pulverized. Grinding the nuts with the motor running in a continuous stream will make them oily so be sure to pulse if using a food processor, or run the motor with an on-off motion if using a blender.".


Note: In a comment someone named Lucie suggested: "To prevent almonds from getting warm and oily during the processing try to freeze them before". David replied: "That’s an interesting idea and would probably work ~!".


Casey Thaler: "Almond Flour: What You Need to Know About This Grain-Free Substitute":


  "Be careful to not let the nuts get oily (at that point, you might want to make almond butter). This can be avoided by doing a stop-start motion with the on and off switch. The continuous heat (if you leave the processor on) will start to make the nuts oily, which will not be ideal when making flour.".


Alice Medrich: "How to Make Your Own Nut Flours (Without Making Nut Butter)":


  "To pulverize nuts in a food processor without turning them to paste, be sure that the processor bowl and blade are dry and cool (not hot from the dishwasher) and the nuts are at room temperature (not cold from the fridge or freezer and not hot from toasting in the oven). Pulse until the nuts are as finely ground as you like, scraping the sides and corners of the processor bowl with a chopstick from time to time.
  
  If you follow these rules, there is no need to add flour or sugar from the recipe to keep the nuts from turning to butter, as is sometimes advised, although there is no harm in doing that either!
  
  p.s. You can also make a fluffy nut flour or nut meal in the processor, using the fine shredding disk instead of the steel blade: the same precautions about the temperature of the nuts and the disk apply.".


Eileen Troxel: "Italian Amaretti Cookies":


  "Combine the almonds, cornstarch, and powdered sugar in the bowl of a food processor. Pulse the mixture until a fine powder. (The almond meal would become oily if ground on its own.) Transfer the almond-sugar mixture to a bowl. Using a wire whisk, stir the mixture to break up any lumps, and set aside.".


Other Info:

In the making of Macarons (almond-meringue cookie sandwich with filling) there are two styles explained on Wikipedia's webpage:


  There are two main methods to making a macaron - the "French" method and the "Italian" method. The difference between the two is the way the meringue is made.
  
  In the French method, egg whites are whisked until a stiff-peaked meringue forms. From there, sifted, ground almonds and powdered sugar are folded in slowly until the desired consistency is reached. This process of knocking out air and folding is called macaronage.[15]
  
  The Italian method involves whisking the egg whites with a hot sugar syrup to form a meringue. Sifted almonds and icing sugar are also mixed with raw egg whites to form a paste. The meringue and almond paste are mixed together to form the macaron mixture. This method is often deemed more structurally sound yet also sweeter and also requires a candy thermometer for the sugar syrup.
  
  Either Italian or French meringue can be combined with ground almonds.[16]
  
  A macaron is made by combining icing sugar and ground almonds into a fine mixture.[17] In a separate bowl, egg whites are beaten to a meringue-like consistency.[18] The two elements are then folded together until they are the consistency of "shaving foam", and then are piped, left to form a skin, and baked.[19] Sometimes, a filling is added. 
  
  [15] "French pâtisserie technique: Macaronage". Le Cordon Bleu.
  [16] "How to cook perfect chocolate macarons". The Guardian.
  [17] "Macaron troubleshooting tips". Better with Butter.
  [18] "Macaroons". BBC.
  [19] "Macaron Myth Buster: French or Italian?". The World of Anges. February 22, 2015.


Le Cordon Bleu: "Making homemade French meringue"

Le Cordon Bleu: "A Fasinating Recipe: Lemongrass Macaron" (Almond meringue with lemon ganache)

Le Cordon Bleu: Video: "Making Macarons with Pierre Hermé at Cordon Bleu" (Italian meringue method)

IronWhisk: "Macaron Mania: The Recipes of the World’s Top Pastry Chefs" compares the recipe and technique of 5 of the world's top pastry chefs.

In all cases the egg is whipped to medium peak and then the dry ingredients are added. Doing it differently will provide a different (wrong) result, you won't have fluffy eggs; the whole point of meringue, and without which you simply have mixed egg whites.