FX is one of those features that simply depends on who's using it for what. I personally don't use them, and don't miss them in MIXXX. But I can see where others would want to have them.
When it comes to sound processing of any kind, I can think of two things. First is compression. Yes, MIXXX normalizes the songs and does a nice job, but there are still times when one song seems a bit louder or softer than another. Compression (if used right) can help stabilize the volume while keeping the sound natural. The other thing might be echo. I normally don't use this, but I can recall years ago when radio station WPGC in Washington made use of an echo in all of their broadcasting. Very subtle, just a tiny amount, but it had a way that made every other station sound flat in comparison. If I had an echo FX, I might experiment with the same type of sound.