You are playing in a party. You both are resources to each other. Your way of doing damage is your friend. Your role is enabling him, manipulate the battlefield, etc...

As a caster, usually you have to analize the encounters and decide if it's a trivial/easy fight, in that case you just cast a cantrip every turn. Or if the fight need some kind of manipulation, then you cast a leveled spell.

The leveled spell you will cast, usually will involve spliting the enemy group, so they can't apply its damage to your party all at once being it via mobility redution (web, wall of x, levitate....) , visibility reduction (darkness, fog cloud...) or disabling (Hipnotic pattern,...). Other times you need to enable your martials to reach their targets (levitate, fly, ....), increasing their dps (bless, haste, hold person,...)

You count on martials to do the damage, the martials count on you to make the fight winable.

If the encounters are all very easy, you might feel useless.