I've got a Huawei watch with a sapphire glass display. It doesn't need a screen protector because the chances are it's a much harder substance than anything else it comes into contact with.
I've been extremely impressed with its ability to resist scratches to the screen, and I've bashed it around a lot. It was literally my first watch ever as an adult, and made me realise just how closely I'd been walking to things around my house and when out and about, because I was constantly bashing my watch into things, yet no damage or even the hint of a scuff to the display.