D&D 5E - Warlock - What is it actually good at?
To make the most out of a Warlock, you need to recognize and take advantage of the two aspects of their spellcasting that are unique among classes:-All of their spell slots are of the highest level they have access to (up to level 5),
-Their spell slots recharge on a short rest instead of a long rest.
A level 9 warlock who gets two short rests in a day will have access to six level 5 spell slots to a wizard's one. To take advantage of this, it's very important to only use spells that take full advantage of being cast out of the highest level spell slot you have. Shield and Hex are great when cast out of a first-level slot, and awful when cast out of a fifth-level slot. This is a problem, since a lot of the Warlock's best spells (Hex, Darkness, Misty Step, Suggestion, Hypnotic Pattern) Don't scale well or at all. That being said, casting Fireball (at level 5) or Evard's Blade Tentacles (At level 7) or Cone of Cold (At level 9) six times in the same day is better than what a Wizard or Sorcerer of the same level can do, in terms of sheer power.
In order to make this work, you need to make sure that you're getting the recommended two short rests (or preferable more) in between each long rest. The best advice I can give for this for this is to Talk to your party and to your DM. Explain to them that your magic comes back on a short rest, and that you'd like to take a short rest to recover your spell slots. Some characters don't gain anything back from short rests, so they tend to forget about them.
You also need something to so when you're not casting your spells, since you won't have enough for every battle. The default choice is Eldritch Blast combined with the Agonizing Blast invocation, which offers the highest damage in the game for a cantrip and scales well. Blade Warlocks (particularly Hexblades) can get away with using a weapon instead (As long as they take the Thirsting Blade and Lifedrinker Invocations)
Out of combat, the Warlock offers similar utility to the Sorcerer: They're a Charisma-primary class that can take make use of the social skills. Since Persuasion isn't on their class skill list, I highly recommend picking it up from a background.
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