Why is the silver salt ammoniated to form the Tollen's reagent?
Yes, it is important to convert your silver ion to diamminesilver(I) complex. If you do not add ammonia to silver nitrate, the silver ion is reduced so quickly that colloidal silver metal would appear and the solution would become a black, cloudy liquid and you won't get the silver mirror when it reacts with aldehyde.
It is also important that you maintain the pH of the reaction. During the preparation of Tollen reagent, you will see the use of sodium hydroxide. This is very important for maintaining the pH. During the whole reaction, the pH is between 8-10 and it goes even faster if the pH is beyond 10. The reaction also becomes more sensitive. This has been explained in details in this journal by going through the exact mechanism of the reaction.