There are a fair few spells that have more powerful results with a higher Caster Level. Magic Missile creates more missiles, Fireball uses more damage dice, Cure spells restore more hitpoints, etc. In addition, any spell that can be resisted/countered/dispelled uses Caster Level to determine how likely this is to happen.
But I'm unclear where exactly this caster level comes from. Lets take Magic Missile as our example. It is a first level spell, so as a level one wizard I can prepare it in a first level spell slot just fine. But once I become a fifth level Wizard, get a caster level of three and have access to third level spell slots, I'm not sure what happens.
If I prepare Magic Missile in a first level spell slot, does it get its second missile because I personally have Caster Level three, or does the spell itself need to be in a third level spell slot to be considered a Caster Level three spell? Or is it some third thing I'm just unclear on?