The glass adds tensile strength to the resin. Resin on its own lacks this strength, although it will bond with the wood which does help hugely.
The glass doesn't water proof, only the resin does that. You can also use other materials, e.g. nylon stocking material works well with epoxy resins, and will be lighter than glass cloth.
Before you steel 'her indoors' stocking and tights remember to ask! I once used one her tights to sheaf a small Riva of 24" length, I then absent-mindedly put the said object on the side board meaning to throw it away. She piled it up and put it away. A couple if weeks later we were going out and she went to put them on to find a big boat shape cut out of them.
1. I got hit
2. It cost me several new pairs
3. It cost me a rather expensive dinner
4. A night in the sofa
5. I got hit again
6. She now checks all my boats and her stocking and tights
7. Every year for Christmas and birthday I get tights as a present.
Moral don't leave the evidence around. You can blamed the washing machine monster.......you know that one that only steals one sock!