Because the muscle has to repair and you need rest. You can use the same machine, but you wouldn't because you'd want to hit the muscle from a different angle in a different way.

All the idiots come to my gym on a Monday, spend 45 minutes on flat bench and then go home and you'll see them again next Monday. Utterly stupid.

Example of a good chest workout:

Incline bench, 4 x 10.
Incline dunmbell press 4x10
Incline dumbell flies 4 x 10.
Dumbell pull over 4 x 12 to finish for a good stretch.

A lot of people think flat bench is the be all and end all, since I started it's been drummed into my head as a dangerous exercise, and no good for size. A lot of people disagreed, but as my trainer was training me, I did as I was told. Read up on Dorian Yates who was number one in the world for 6 years, and he agrees with my trainer so as far as I'm concerned, it's happy days.

So my advice is stick to incline, but some will disagree. Decline too is very underated, but don't over complicate it.

Benni.