There are a few good reasons to use a DI:
1) If the DI signal is picked off before the amp input, the amp can be distorting, crapping out, on fire, but the FOH will still have a useable signal.
2) Using a DI excludes any sympathetic rattles, buzzes and vibration from the cabinet or nearby objects getting into the FOH send.
3) Better isolation and lower noise floor.
4) Less chance of the bass player messing up the mix midway through a performance by knob twisting on the amp (although if the bass is too loud through the amp, it messes everything up.)
5) Much more flexibility in EQing the bass send signal, as it isn't pre-EQd through the amp and speaker. I know many advocates of micing a bass cab will argue, but the mic'd cab tone on stage isn't always the best tone for FOH.