lets take some examples:
say you want to add a reverb to a track…if you insert the verb on the track, you’re limited to the processing in the reverb plugin itself (filters, maybe some simple eq). if you were to put the verb on an fx bus send/return, you can follow it up with your favorite eq plugin to shape it with more detail (low-cut filter out the rumble, eq out mud or unmusical frequencies, brighten it etc). in Ableton Live, you can get around this by inserting an FX Rack on the track and make your own custom chain.
the same goes for parallel compression…a lot of times it’s nice to run the compressor in parallel with aggressive settings and eq the compression (try -10 to -15db around 400-500hz with a wide Q). I don’t know of any compressors that have that sort of eq onboard so you’d have to run it as a send/return or use the FX Rack in Ableton.
sometimes it’s fine to just insert a reverb with an internal lowcut filter to keep it simple and work fast but eq'ing your verbs can really clean up your mix.
also this can be applied to any processing, not just verbs and parallel compression.