Aquaplaning is one of the most dangerous things that can happen to vehicles on a road, because the tyres lift off the road and so have no grip. Under these conditions, there is almost no friction, and so the vehicle cannot stop.

The deep grooves in tyres are there to throw the water away from the tyre and so give good contact with the road. When those grooves become shallow (because the tyre wears away) they cannot do their job properly. This is why tyres need replacing before the grooves are worn away.