Yes all of these effects require that you effect the whole signal.

There is absolutely no point in applying compression or say a noise gate to only 25% of the signal.

Putting effects in your loop is all about only sending some of the signal to the effect, hence the controls on you amp for mixing: A) how much signal goes down the loop and B) how much of the signal that comes back gets mixed in.

Placing things like a wah / compressors and noise gates between the guitar and the amp allows you to effect 100% of the signal.

However things like flangers, choruses and phasers and even delay pedals work really well in loops.