Attributes influence the damage/healing/effect of the skills they govern, and in addition to this Intelligence reduces the cooldown of magic skills. Abilities govern the number of skills and the surcharge paid for using higher level skills.
Generally speaking, for each skill category:
Ability 5 is for the perfectionist that wants to know all skills even if he uses only a small subset of them most of the time, for the player who hates the idea of paying extra AP for any skill, for the player whose character is specced such that he wouldn't have enough AP to cast the highest level skills otherwise, and for the player that wants one of the 5 point talents. (Given how poor most of the magical 5-pointers are, this last one strikes me as a remarkably poor reason where the magical skills are concerned, while, on the other hand, given just how good the non-magical 5-pointers are, going from 4 to 5 is highly desirable for them.)
Ability 4 is for the player that wants to have a great skill selection but is okay with paying a small surcharge of 2AP for the highest level skills (17-20), while paying regular costs for the most frequently used skills. This is an attractive end point for many magical characters due to the lackluster 5-point talents, and you'd need a really good reason to go further with a magic-using glass cannon character, who can always find something to spend attribute points on - survivability (Man-at-arms with picture of health talent) if nothing else.
Ability 3 is for the player that wants to have a good skill selection but is okay with paying a small surcharge of 2AP for skill levels 13-16 and +4 for 17-20, while paying regular costs for the most frequently used skills. A glass cannon mage, for instance, is likely to find this a nice place to stop for all but his most frequently used school of magic.
Ability 2 is for the player that wants just that little bit more than skill 1 provides - lowlevel damagedealing skills a surcharge, for instance, to ensure that elemental damage of all types can be realiably dealt in case of facing enemies with specific vulnerabilities or resists. You'll pay a surcharge of +2 for 9-12, +4 for 13-16, +6 for 17-20.
Ability 1 is for the ability to deploy a few powerful lowlevel buffs at base cost (Oath of Desecration, Wildfire, etc.), for emergency healing, or for seldom used higher level spells at exorbitant costs. You'll pay a surcharge of +2 for 5-8, +4 for 9-12, +6 for 13-16, and +8 for 17-20.