Simplified reps and layers both can be used to display or hide parts. The difference is layers turn off display only. Simplified reps dumps the file from memory or does not load the file in memory initially. If you create a rep that turns off a bunch of pipes and cables and springs, you can regen much much faster. (dont forget to erase part from memory) If you turn them off with layers it still regens them so you decide what your performance will be. Personally I only use layers for datums, curves, axis, planes, notes...stuff. I have a list of reps so I rarely open the master rep.