Among other things, the technique is psychologically relieving to those people who don't trust their own ears/judgement/skill with compressors.
All else fails, "sneak a little" of the un-destroyed signal in. I mean, the fidelity can't be all gone, right?
It became very trendy in the last few years and has resulted in a lot of weird-sounding, slightly smeary (as Brian says above) music, where you can't quite put your finger on what is wrong exactly. Just doesn't sound very good.
This kind of insecure mixing is in contrast to great masters (and celebrated eccentrics) like Tchad Blake, who use the technique daringly and "painterly" with a sense of purpose and design. This is the way to use it. Like an artist.
Unfortunately, the spirit behind it is often halting/hedging/timid/over-careful.
Generally I encourage people who are new and learning
not to resort to this degree of complexity
first. Try to get a real command of attacks and releases and ratios and different topologies of compression
first. Get the basics down. This can take many, many,
many years! Arguably
a lifetime of learning, the way people study the sitar (or architecture) all their lives and don't consider themselves masters until they're in their 70s. Y'know? A lot to figure out before you start gettin' fancy.
Sense you're a student, it might be interesting to try to research/discern what the earliest uses of deliberate parallel compression were in the history of recording. Me, I really don't know. No clue. Were they using it on Beatles records? I don't think so. But I'm not an expert. My sense is it's a fairly recent phenomenon in the (still young) history of recorded music.
cheers,
c