(Author here) It's useful to render things like gamepad buttons, in-game items and characters, etc. Here's an example from Nintendo: https://scirra.aha.io/attachments/7235903469701444113/token/...

Another point to note is this is rendered in to the canvas so it can be processed with effects and layered easily with other game content, which isn't so easy with HTML (although our new HTML layers feature also helps with that).