User blog comment:Miskos3/Fairy Tail 521: The Strongest Wizard, Review/@comment-5894890-20170212010455/@comment-7418318-20170214182223

I believe that in this context MP is just the magical reserve. To put it in game-mechanics terms it is Mana, not as you seem to think the Intelligence stat.

If they have more MP it only indirectly boosts their abilities in so far that they can afford to pour more magic into each action without having to worry about running dry or use spells that consumed too much magic for them to use without the boost (like Nakagami armour, Wings of Love, Dual Summoning, stable LFDM, ect).

It does not by default make them physically stronger for example or increases the power of the spells if they put as much magic into them as before.

If for example there was a cap on how much "mana" you can pour into spell X and you already have enough total reserve to cast it at it's full potential, then increasing your total reserve would be useless for this particular spell, though you could use it to improve others or learn more advanced spells.

At least that's how I interprete the whole matter.