Intelligence determines how well a character learns and reasons. Intelligence is important for wizards because it affects how many spells they can cast, how hard their spells are to resist, and how powerful their spells can be. It is also important for any character who wants to have a strong assortment of skills; however increasing intelligence will not grant bonus skill points retroactively.
The base intelligence score is used to determine the highest level spell a wizard can learn. This maximum spell level is the score minus ten.
The intelligence modifier is added to the skill points gained each character level (subject to a minimum of 1 skill point per level) and to the DC of spells cast by a wizard. In addition, wizards gain bonus spell slots based on this modifier; for details see ability modifier. Furthermore, the intelligence modifier is added to the appraise, craft armor, craft trap, craft weapon, disable trap, lore, search, and spellcraft skills.