What are the uses of mild steel and stainless steel?

All manner of uses!

Mild steel, without or without alloying additives as well as the carbon, is the main engineering material; but it rusts so stainless-steel is used when corrosion-resistance is important but the item cannot be painted or plated, such as food-processing equipment.

Certain classes of stainless-steel are also nearly non-magnetic, also important in some applications.

There are also very many different alloys of stainless-stell, developed to have particular characteristics such as good weldability, high ductility, easy machining, etc.