The courtesy is as simple as leaving the toilet dry and tidy for the next passenger. The reasons are:
- Dry lavatories look cleaner, while wet lavatories even when are clean they look dirtier.
- Leaving the basin dry and tidy is indeed a courtesy to passengers who put their stuff next to the basin (mobile, watch, etc.).
- Flight Attendants can not just leave everything and get busy wiping water off basins, so it is better for passengers and airlines, because people will simple blame the airlines if lavatories found wet/dirty.
- The more wet the place is, the more better it is for bacteria to grow, and vice versa.