The main reason for equipping a Job to your retainer is simply to allow them to equip Job-specific gear like AF, relic weapons and tome gear - remember that the likelihood of what your retainers find during a venture is directly determined by both their level and the ilevel of the gear you have equipped to them.

If you only keep your retainers geared in low level stuff or even don't bother equipping anything other than a level 1 weapon will just mean they will only ever bring back worthless junk like a lump of mudstone or a harbor herring. The higher their ilevel the better the chance of finding rare items, and thus setting a Job to them only helps that.