By expending additional charges, you can use a wand as if its caster level higher than its normal level, up to your caster level, changing all the spell's level-dependent effects.

For example, by expending 3 charges at once, a wand of magic missile (created at caster level 3rd) can be used at caster level 5th, firing three missiles instead of two. If you expend 5 charges at once, a wand of magic missile (created at caster level 3rd) can be used at caster level 7th.