The best use is to pickup spells that you otherwise can't access. Except for Guidance (you can never have too much Guidance), you should pretty much only consider spells the rest of your party can't cast and that aren't on the Warlock list. I'll break them down into utility and combat, since you'll probably want one of each (plus Guidance... have I stressed how awesome that is enough?).
Combat
Vicious Mockery - very little resistance/immunity to psychic damage, disadvantage is super strong until opponents get multi-attack
Sacred Flame - might be useful against low dex/high ac opponents, but in general Eldrich Blast is going to be your go-to damage dealer
Thorn Whip - pretty solid control, and since it may not come up often, shouldn't fight with EB as your primary attack
Shocking Grasp - kinda weak for your needs, but sometimes a get-out-of-melee free spell
Utility
Dancing Lights/Light - unless the party all has Darkvision, I've found that one of the spells per group is a good idea
Mending - fixing broken/damaged things, assuming you have a flexible DM, can provide a lot of extra information when delving old tombs and such
Message - I never see this come up much, but it fells SOOO good when it does
Resistance - not a fan of this spell, because you almost never get a chance to cast it before you need it
Did I mention Guidance?