Darkness: Useful for solo missions
The party I currently run has a warlock with darkness and Devil's Sight.
Unlike Dale M, my party doesn't use a magical light source. Rather, they have three of six members with racial darkvision, and they use the Darkvision spell on two of the humans (the third human is the warlock with Devil's Sight).
While the advantage / disadvantage mechanic of fighting in magical darkness can be powerful for the warlock in melee, in practice it is not used that often.
The principle reason is because it is just too costly. Even at 10th level the warlock still has a meagre two spell slots, and using half of them on a darkness spell is seldom worth it even in the most opportune of conditions.
Early on the warlock was pact of the blade, but quickly realized that he did not want to hang in melee with the party tanks and take as much damage as they were. (The tanks are a wolf totem barbarian who is a damage sponge / advantage generator and a champion fighter with maxed Con, magic plate, and AC20).
Since the warlock switched to pact of the tome, generating advantage for himself in melee is seldom desired, and generating effective disadvantage for others to hit him is easier done by staying at range, using cover, and being prone (none of which cost scarce spell slots).
Even when the warlock was experimenting with being a melee fighter, though, constrained battlefields often meant that his darkness interfered with the others' sights. Since there were more of them than him, and since the dedicated melee characters hit harder than he did, that ended up being a net loss for the party.
The darkness is occasionally useful, but it is situational.
One time the party found themselves in a close range missile ambush, with the enemy behind hidden and fortified positions. In this case the warlock put up darkness, and the other party members entered it to protect themselves until they could formulate a response.
Other times the warlock has used darkness is when he is forced to be solo. He is not the party scout (that is the Stealth-maxed lore bard), but when he finds himself on his own, he can profitably use darkness to both avoid detection and to protect himself if detected.
Sometimes he is charged with holding the flank or rear of the party when they need to restrict access to the battlefield to prevent reinforcements from arriving. Since he will be fighting alone, that is a great time for him to whip out the darkness.