I'm not fond of warlocks because I'm not down with the fluff. But mechanically:

1. they're the only caster whose main attack cantrip is on par, or nearly on par with the fighter's weapon attacks without depleting resources.
2. Eventually you get 4 spells per rest that are cast or upcast to level 5. That's significant magic power brought to bear.
3. They eventually have 6-9th level spells. Think it's just one and mostly once per day, but still.
4. They eventually get the ability to spend something like a minute and get the benefits of another rest every day.
5. Choose the right invocations and you get a significant number of lower level/utility spells that you can cast or have the effect of without restriction.