How often do you change your workouts...?

[EnderJE]

I've heard that you should change the workout every so often (4 - 8 weeks) or after weight increases are no longer possible.

I assume that change is either a new rep scheme, new exercise, new split or new TUT.

Wouldn't it be better to lower the weight down to 80% of 1RM and start again? For example, I love to Squat and DL...why would I switch to another exercise?