It does depend. When I was a consultant, we tried to charge based on value (by engagement). OTOH, clients often still thought in terms of hours. If we were on-site for one engagement that was only going to take half a day, they'd often try to hook us up with some other group as well given we were there and had the time. (Typically we'd go along as these were long-time clients.)

And some areas like law are very hourly-based. The one time I did some legal work it was strictly based on time. (At a very good hourly rate including travel time.)