Magic Boost is an offensive attribute which increases a character's ability to inflict damage with magical attacks and spells. Sorcerers and Spiritmasters benefit the most from increased Magic Boost because they rely on magical spells to a large extent. However, Clerics and Chanters can also benefit from increasing their Magic Boost. This attribute is of very little benefit to melee classes such as Assassins and Gladiators. However, some Assassins might consider going with a full magic boost item build due to a number of their skills scaling on it.
Magic boost works like a damage modifier. On the mage classes, 12 magic boost equal 1% increase of magical damage. The scaling on a warrior is set to 7 magic boost being the 1% magical damage modifier. A templar has two skills that scale off of magic boost and it would be realistic to do a pretty good sum of magic damage if this is stacked to a crazy extend, yet very unreliable. The assassin has a much better skill kit that actually enables them to stack magic boost and have good results.