Yes,
It adds to the range of possible MP increases you can get for every 10 INT you have (if you have 29 INT, say you gain 5-8 MP per level. If you had 30, you'd gain 6-9.), and
Put into DEX. All the guides about adding to HP come much later, in 4th job, when DEX is worth much less with respect to WATK or skills.
I do not know if it's 10 INT = 1 MP or 2 MP or what, but I think it's 1 MP.