Just what are seatbelts for??

In Phuket, our tourist brochure warned that helmet laws for motorbikes are strictly enforced. We saw that was the case. Coming up behind a motorbike pulled over because the driver wasn't wearing a helmet, our taxi driver slowed and sneakily fastened his safety belt. He looked back at us and said something about police, so I guess not wearing a seatbelt as the driver is also illegal. We slowly drove past. But once clear of the pending danger of a ticket, he again unclipped the belt, sped up and went on his way. I wonder what he thinks seat belts are for? Observing the motorbike riders... it seems only the driver is required to wear a helmet. The three year old in front of the driver and girl riding side saddle on the back don't need one. On every motorbike in Phuket there are two, three, or more people, and very rarely is anyone but the driver protecting their head. Seems like not much is accomplished by the helmet law.