Honestly, not much. Any of the Brussels museums should be just walk in, there are some good ones, but all are a bit niche, and do not see huge crowds compared to those in Amsterdam, Paris, Rome, etc.
In Ghent, if you are interested in seeing the altarpiece, The Adoration of the Mystic Lamb, at St. Bavo's then I would l would visit the website and likely reserve a time.
Bruges, I can't think of anything that requires it.
In general, if you are interested in a venue, check out their website and see if they require prebooking, but if they do not, then really no advantage in doing so, unless you have a set schedule.