My guess (and that's all it is, I'm no expert on this) is that with a rounded blade only a couple teeth are in contact with the wood at any one time so there's less resistance/friction. it's a lot easier to pull, say, 3 teeth through a piece of wood at once than 10.
Crosscut saws tend to be pretty long and are for cutting through pretty sizeable trees/logs, and if the wood is big enough the entire height of your saw is probably going to be inside the cut so that's a lot of potential for all kind of friction and pinching, so I'd imagine every little bit helps. It's probably less of an issue with regular hand saws and smaller lumber.