The warlock's biggest advantage is no real limit on the number of times per day he can use his powers. (He's got a couple of powers with limited uses per day, but 90 percent of his powers have no such restriction.) The thinking here is that in most D&D [sic] games, your characters are probably going to be in only 15 to 20 rounds of combat between rests and spell recoveries. So after your spellcaster has a total daily spell allocation of 20 spells or more (say, around 5th level), his real limit is the number of actions he gets per day — the number of specific opportunities he has to cast a spell. So the warlock is still bound to the same ultimate limit that any moderate-level wizard deals with. Now, it's pretty useful to never run out of attack options, and the warlock can blast you over and over again with his eldritch blast. So what he gives up is spell versatility. The warlock knows only a handful of different tricks. On the bright side, the tricks are all spooky, creepy, and oozing with flavor.[5]
https://en.wikipedia.org/wiki/Warlock_(Dungeons_%26_Dragons)