Should a tooltip hold information people need to finish a task?

No. If someone needs a piece of information to get through a task, put it somewhere they can't accidentally make disappear.

Why? Because people dismiss tooltips by reflex. As Tanner Christensen put it from his time at Facebook, people very often, instinctively, dismiss tooltips, on occasion even accidentally, and by the time they realize the message may have been helpful, it's already gone with no way to recover it. Lisa Duddington at Keep It Usable sees the same thing in research: task-focused people avoid overlays, and when they later need a function, they can't find the help and don't recall seeing it.

It makes sense when you think about it. When people are focused on a task, they assume overlays are there to distract from their core flow rather than support it. And making the tooltip louder with bright colors and odd shapes backfires, because anything that looks too different from the rest of the design gets perceived as something that doesn't belong (like an ad) and dismissed.

So where should it go? Right in the interface. I've been saying this about forms for a long time: help that's visible and adjacent to a data request is most useful. Password requirements sitting right under the password field, for example, or an availability check that shows up as you type a username.

The same goes for onboarding. Compare a tooltip overlay with instructions built directly into the screen, like Pandora's "Add your favorite artist or genre to get started," which feels like a cohesive part of the experience instead of something to swat away.

Tooltips still have a place for nice-to-know stuff, like cueing what will happen if you click a hovered object. Put simply, if losing the message would block someone from finishing, it belongs in the UI where they're already looking.