Why do shared apartment homes have two sets of individual circuit breakers?
Circuit breakers protect the wiring in the home from starting fires due to overheating, because when a fire starts in the walls, you can't put it out because you can't reach it, and the house is done for.  Overheating is caused by overload.   The purpose is not to punish people who enjoy electrical appliances.  

It is not the number of appliances, but the sum of their current draws per circuit. You can't run 2 kettles, but you can charge 100 iPhones. 

(If you want to wire a kitchen so you can use 2 kettles, coffeemaker, hot plate, grill, sous vide, toaster and microwave all at once, that's no problem, just wire a separate circuit to every socket.  That's why they make 12/2/2 cable.)

Why are there separate breaker panels? 


To avoid Tenant A having to ask Tenant B to reset their circuit breaker, because that begets ugly hacks like extension cords under rugs, which begets house fires.   The Electrical code is written by the National Fire Prevention Association. 
So that Tenant A cannot shut off Tenant B's power, either by accident or as a prank.   
In many jurisdictions, it's a legal requirement that loads in commons space must be separately metered, i.e. One tenant can't get socked with the bill.  A lot of legacy installations are grandfathered, but then, they have to fix it when they remodel.