I'd nuance this by saying that often it's more important to just set some kind of goal than to know what you want. Very often, the whole point of a gap year is to figure out "what you're going to do" and figure out "what you want to get out of it."
In those cases, a gap year is better than just plowing ahead with whatever you're doing (and don't want to do).
Right, right. That's really what I meant. When I said "know what you want", I meant "know what your goals are," even if your goals are vague, like "explore [X] career field", or "learn about [Y]."
For example, a good gap year would be a full-time (or perhaps paid internship - unpaid internships are usually crappy and pay-your-dues type affairs) job at an interesting company in a field you're interested in. So an example would be working at a startup as a software developer or even a business role.
Ah, yeah. I do agree with that. A gap year with some kind of plan is (usually) preferable over just a gap year, but just a gap year is still better than staying on automatic pilot.
If you're going to take a gap year, you should know what you're going to do and know what you want to get out of it.