There are two main reasons to put compressors on individual tracks:
Compression can change the tonal quality of the track (depending on the settings). Compression is a change in gain with time and signal amplitude, which makes it a non-linear process. Similar to how a tube amp just on the edge of clipping can change the tonality a compressor with fast attack can give a similar effect.
The second is that if there is wide variation in level between the tracks, sometimes it is easier to put a compressor on each track and then set the levels for the blend you want, rather than attempt to get the blend between tracks you want, but then have to add a lot of automation of the levels because of uneven dynamics of the individual tracks. Ideally the arrangement and performance would take that into consideration and the tracks would be recorded with dynamics that work together, but that doesn’t always work out for various reasons.