PUSH and POP are very usefull, when you are in the interrupt routine. Int. routine (f.e.from Timer) interrupts your main program and (depending on programming technics) destroys register you are currently using (because usually you have to use some of registers in the interrupt routine too). So at the beginning of interrupt routine you can PUSH registers and program status word to ensure, that after returning from interrupt and POPing registers, program is working in the same way (like before int.).