The point of a highback is to be able to apply pressure to your heelside edge. The more forward lean you have in your highback, the quicker you are able to push your board onto your heelside edge. If you are traveling at higher speeds and carving deeper dynamic turns then you would want more forward lean. In the park you don't want any forward lean. Why wouldn't you want any forward lean in the park, you ask? Think about it, when you're sliding a box front-side, do you want your heel edge to be innitiated? Nope, unless of course you want two broken arms and/or wrists. Having your highback straight up allows a lot of movement before engaging your edges, which in turn is more forgiving in the park. This is the same reason why park boots and binding have a lot of flex- to allow more forgiveness.
Hope that helps