SLOP...never found it necessary
Ah, but you
were doing it in the past, in pre-GPS days. The lateral navigation errors inherent in the old systems added a bit of "dither" to your position. Now we all fly *exactly* the same track between waypoints and if someone is coming the other way at the wrong level using their super-accurate RVSM-certified altimetry, you're almost certain to collide. An offset to the right mitigates against this (admittedly rare) scenario.
Why not to do it? Can't really think of any good reason...