Purpose of condensed milk in mango ice cream?
The ultimate goal of ice cream is a creamy texture. You could take those same ingredients and freeze them in a paper cup and make a fantastic mango popsicle. Popsicles and slushies are not nearly as magical as ice cream.

Creaminess in ice cream is achieved by keeping the ice crystals as small as possible. The bigger the crystals the more grainy the texture.

There are two good ways to prevent large ice crystals from forming: Reducing the amount of available water and mechanically destroying the crystals. The ice crystals are reduced mechanically by churning or in your recipe by blending- but this is an answer for a different question.

Reduce the amount of available water.
 Many ingredients in ice cream will tie up some of the water and prevent it from coalescing into monolithic crystals.


Alcohol and sugar are used in sorbets
Gums, such as carageenan and xanthan gum, are used in commercial ice cream
Egg custards are used in French style ice cream
Starch gels tie up free water in Italian Gelato
Large quantities of fat


Removing water from the recipe, and sometimes replacing it with fat, will go a long way towards making creamier ice cream.

The condensed milk in your recipe is a convenience. Using milk that has had a lot of the water removed means that there will be more fat and less water in your ice cream. This will make it much easier to make your ice cream creamy. 

You could use regular milk but you would be relying on your blender to chop up the large crystals that would inevitably form. If the canned milk is too expensive, and if you have some time, you could reduce your milk yourself and get the same effect.