If velocity is 20, then 20 points worth of stories fit into an iteration (week). Points include testing, so that isn't cut out, like with sprints. As a PM, I can get developers to point stories out for a few weeks worth of iterations. If I need to move stories around, I can do that easily and I can see how moving things around affects 'when it will be available'.
If I was going to use another tool, I'd want PT's 'velocity' as a base feature for sure.