Warlock/Druid seems a lot weaker that single-classing in either, so the idea is mainly for the flavor/RP/whatever.

E.g., Warlock 5/Druid 5 has ton of good spells Level 3 and below, as well as some Wild Shape. But Warlock 10 would have Scrying and Contact Other Planes, which I think would be better at information-gathering than all the cool infiltration stuff Druid adds. And Druid 10 is clearly better in spell slots -- this of course isn't like 2 ordinary full caster classes multiclassing -- as well as having flying wild shapes, so is probably quite the competitive infiltrator as well.