Equalizers and compressors units work best when you process the entire signal, so they are commonly used with inserts. The reason for compressing a vocal track, for instance, is to limit its dynamic range. Unless all of the vocal's signal is sent through the compressor (through a channel insert, perhaps), a portion of the signal will retain its original dynamic range, thus preventing you from keeping a firm lid on loud passages.

Instead of using the channel insert, you can patch the compressor in-line with an outboard mic preamp while tracking the vocal or route the output of your recorder through it while mixing. But if you need a mixer's pad and input preamp to optimize the signal level before compressing it, the channel insert is the best place to patch the compressor into the mixer.