Why do we "PUSH EBP" and "MOV EBP, ESP" in the CALLEE in Assembly?

The given answer from Remy is perfect, however here is one small addition, a thing you might also see right after

mov ebp, esp

it's very possible to see instruction such:

sub esp, 20h   ; creating space for local variables with size 20h
sub esp, CCh   ; creating space for local variables with size CCh

along side with an AND call sometimes (like and esp, 0FFFFFFF0h). This is also part of the dealing with the stack and it's done so the stack can be align and be divisible by 16. Of course all this depends on the used calling convention (cdecl, fastcall, stdcall etc.)

It is establishing a new stack frame within the callee, while preserving the stack frame of the caller. A stack frame allows consistent access to passed parameters and local variables using fixed offsets relative to EBP anywhere in the function, while ESP is free to continue being modified as needed while the function is running. ESP is a moving target, so accessing parameters and variables using dynamic offsets relative to ESP can be tricky, if not impossible, depending on how the function uses the stack. Creating a stack frame is generally safer, at the cost of using a few bytes of stack space to preserve the pointer to the caller's stack frame.