Compression is an invaluable tool for audio engineers. Despite having been engineering for over 15 years now, I feel as if I’m still learning new techniques and applications for compressors on a regular basis.
Compression is a great thing for gluing a mix together, if used correctly. Not everyone uses it correctly.
Compression is also great for adding punch or slap to drums, bass, and other instruments, smoothing out a vocal that’s all over the place, or even getting that smashed in-your-face radio voice and old Disney vocal sound (e.g. Bear Necessities). It all depends on how it’s used.
Compression is difficult to master (no pun intended). See Dave Pentad’s site for excellent information and tutorials.