You probably find it irritating. Someone close you to you - a friend, family, a work person - is clearly trying to find something. You want to help (or to stop the restlessness) so you ask helpfully "What are you looking for?" and the response is some variation on "I'll know it when I see it."
How can you look for something if you don't even know what you're looking for. You can't. That's why you can't help your "I'll know it when I see it" friend. You can, however, look to find correspondence between a feeling you have and something outside yourself. In fact, that unspecified drive can be particularly powerful.
There are two huge benefits to "I'll know it when I see it." The first is that you filter out much less information than usual. When you know what you want, everything is either "something like what I want" and "not what I want." This isn't possible when you don't know what you want yet, so you are forced to look at everything and ask "are you what I want?" It's a little like the children's book "Are you my mother?" You get to meet the most interesting collection of people and circumstances when you are not sure which one is already what you want.
The second benefit is that it is very irritating to know that you want something and not know what it is. And that irritation is a great motivator. You really want to solve this puzzle and so you look more places. Or you put the search on the back burner, and then find it is surprisingly resilient. You just keep looking for that thing you know will satisfy your very specific, unspecified want.
When what you want is something new, something that widens your frame, something innovative, try telling yourself, "I'll know it when I see it." Then go hunting until you can find something outside you that integrates perfectly into the pattern of what you want.