Loop for time and modulated effects like delays, etc. Some claim to like it in front of amp...however, in front its going thru both power and pre sections which can really alter the 'feel' of an fx pedal and add unwanted gain and grit.. it won't sound as warm and full as if in loop. In the loop the fx signal comes b4 the preamp tubes and returns after pre and before powertubes... which works better for delays and other modulated fx.
I have a TC Flashback in my loop (parallel) of my Gower Killer Kali++ and I love it. Only effect I use.