It's a holdover from 1.0 when you had limitations on stacking stats (couldn't go all in on a single stat), you had a secondary damage stat (don't know them all but MNK was STR/INT for damage), and if I recall almost everybody needed to allocate at least some VIT to not have terribly low HP. On top of that many more skills used MP so piety (or was it mind at the time...?) might be needed to some extent even for non-casters. More stats also did double duty for some things as well I think (Dex contributed to accuracy and/ or crit I think, mind or piety also contributed to magic accuracy, etc.).

As it is now, it barely makes any sense and for most cases may as well be absorbed into the stats you get automatically from leveling up.