Main reason for a side exit is less weight, cheaper, and potentially less restriction for the bore size due to shorter length.

As well as it looking the bollocks, of course

Depending on the track, a side exit can HELP you get past noise limits as the exhaust may well be facing the opposite side to the micraphone that picks up drive-by noise limits

Get a custom 3in side exit made with two silencers, should pass noise regs anywhere and no restriction on a RST