This way the weight of the saw and therefore the cutting force will always be concentrated on a small number if teeth, which are able to slice deeper thanks to the extra force. Remember that when crosscutting you need to slice wood fibers. Rather than shear them as you do when ripping.

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.