Someone probably has a more professional way to break this down but I'll give it a shot
You quote hourly because that's the standard (Your client asks how much it costs to work with you and you reply X$ per hour) but you bill in increments [either fifteen, or some people bill on a six minute increment, etc.] because maybe a project takes one and a half hours. Or, one hour and thirty two minutes [you'd round down to 1.5 hours]. You don't charge for two full hours there, so it's important to have some incremental way to bill. Some VAs do charge by the minute.
Does that help at all, or did I confuse you more?