Like whether or not she likes fishing.
More than that. If she
doesn't like fishing, you will find out whether or not she's willing to go with you anyway, just to spend time with you, and how much she's willing to invest in something you obviously enjoy. If she
does like fishing, you spend a weekend together doing that (and perhaps some other things) and again you learn more about each other.
Joe's right; you can potentially find out a lot which can apply to the current relationship.