Switches on at wall switch rather than power button on the front of the PC?

Could you please tell me why my PC powers up and boots when I switch it on at the mains plug on the wall rather than the switch on the front of the PC?
I can switch it on from the front button, providing I haven't cut the power to the power supply IE switching it off from the three pin plug on the wall.
If switched on from the wall then the front button on the PC becomes superfluous, all I have to do is switch it on at the mains plug and the PC will boot up.
Surely this can't be right?