Just like with weapons, the skill ranks in directed spell indicate the ability to strike harder, faster and with skill.
Thus, the level determines the strength of the attack, with more powerful items providing more powerful attacks. Or, for most daily items and charged items (unless charged staves are common in your games), these attacks via item are a nice powerboost at lower levels, but since wands are either level 3 or level 5 (depending on if they are imbedded with a level 1 or level 2 spell, like shock bolt, sleel V, Confusion, Limb Break, etc), the PC will soon surpass the power of his spell items (and as a neat culture predjudice amongst spell users, having, depending or using such items can be seen as weakness and be frowned upon or schoffed at). That the wands and many daily items become a moot point after a while is a nice change of pace from other games, were more and more gear is needed to be successful.
lynn