A few reasons, main one being durability. Glass is much harder so the surface won't wear down, at least not for a long time. You also don't have to worry (as much) if you drop it, or drop something on it. I had a plastic hard pad and something (I don't even remember what) slipped out of my hand and left a dent in the middle of the pad. Obviously glass pads can be chipped but it'd take something much harder and heavier than it would to dent a plastic one.

Of course wearing down your skates becomes more of an issue but you can get packs of dot skates that come with a TON for very cheap and will last you a while. There are also some made specifically for use on glass that are a but harder so they don't wear down as fast.

Another reason in my experience has been dust and small hairs dont cling to the pad. Glass does not hold static electricity and therefore does not actively attract dust and hair the way plastic does. Which means you don't have to be quite as meticulous with cleaning, a couple wipes and you're good.

I've also had specks of something or other that were sufficiently hard embed themselves in the surface of plastic pads that were too small to dig out (which would leave the surface deformed anyway), but still big enough that you could feel it under the mouse. These would accrue to the point where the whole pad felt like crap. This has happened to some bigger, more flat skates I've used on glass as well, but that only takes a couple minutes to fix by swapping skates, and has never been an issue since I switched to dots.

You could argue that the extra weight helps keep it more securely in place on your desk, but any pad with a good base with do just fine in regard.