Glass can mean see through glass (like in a window) or it can mean glass like in a mirror. Glass sliding closet doors usually mean the latter and they are great for a couple of different reasons.
If you have a small bedroom or any kind of room, having mirrors on the wall help to create the illusion that the space is much larger than it really is. You have probably at some time, walked into a room that had a wall totally covered with a mirror and had to do a double take when you realized you were in a much smaller room than you originally thought.
Glass closet doors can help greatly with this illusion and that is one of the reasons they are great for small bedrooms. You can have either a closet sliding door that open like the picture below:
The only drawback with closet doors that slide open and closed is the rails that are above and below the doors. You have plastic pieces that slide along those rails and sometimes they break or become detached. This can be an annoyance and if that happens, you will have to fix the problem before the door will work. The rails also have to be aligned perfectly with each other or else pressure will be put on the door that might make it be noisy or create friction and not glide smoothly.
Besides those minor drawbacks, glass sliding closet doors work well in many types of rooms but you need to make sure they don't look out of place. Glass can look great when placed in the right spots but it can look terrible if it just doesn't fit in with the decor of the rest of the room.