By the time you get to mastering, your mix should be sounding the way you want it. So a mastering compressor shouldn’t be necessary to “fix” dynamic errors in the mix. But you might feel that the various instruments feel a little “disconnected” — as if they’re all playing the same song together, but not necessarily sitting in the same “sonic space.” Or you might feel that when your track is converted to a compressed format for streaming and download that it doesn’t quite hold together the way you wanted it to, because it’s either not loud enough, or the loud and soft passages make encoding more challenging. Mastering compression to the rescue!
Mastering compression, even more than track or bus compression, is a “game of inches.” You’re not making big adjustments, you’re making the absolute minimum adjustments necessary for the result you want. If you’re using a mastering compressor to “glue” your song together into the same sonic space, you want to use just enough compression to smooth out the loudest and softest peaks, and if you’re using a “color” compressor, just enough to impart some of the sound of your compressor. You don’t want to squash your already compressed mix. And if you’re compressing for the sake of more consistent digital encoding, it might take trial and error at first, but you’ll want to only compress your tracks until the encoder is happy, leaving in as much dynamic range as you can get away with. Mastering is a very specific art, and properly using mastering compression is a big part of it.