“The giants are coming!”
I’m a sucker for classic monsters. Giants are classic example of this. They’ve factored into all kinds of stories in all kinds of media.
Generally speaking, I only use giants as a threat in my games. Without really ever being present, giants always seem like something horrible that could befall the PCs, as well as the NPCs. Its a way to spur action, and cause tension. The beauty is that the giants never even really need to be there. They can comfortably exist as a far-off threat. Something that would require its own special quest to resolve.
On the other hand, they can useful NPCs. Giants can help as well. They basically humanoids, albeit on a very large scale. Maybe a hill giant is having some trouble with something, and “asks” your characters to help it. Why not? Its a reliable a quest hook as anything else, right?
Then there’s the fact that giants a basis in folklore. This is another reason I like classic monsters. Anything that has a history in tales from different cultures can add a lot of depth. If I really want to go this route, then I can dig up some folklore, and tie that into my games. I’ve always felt being able to do this really enhances what can be brought to the gaming table.
Giants are cool creatures. They offer a lot to games, and have an incredible depth in folklore, myths, and all manner of tall tales. They really only enhance games. (Pun may or may not be intended.)