Compressor question

I know it has been said that compressors are preferred "in front" of an amp rather than in the loop, from a sound perspective.

Is there a reason, such as how and/or where it is "inserted" into the circuit? It seems that there are many rack-type compressors which offer great flexibility which could easily be placed in the effects loop. I have no expertise in this area but have heard about parallel or series effect loops, SS vs. tubes, etc. How does this play into it?

Any thoughts? TIA.