Originally Posted by blackllotus

Bottled water is really useful on in lie-flat seats because it drastically reduces the possibility of spilling. It's easy to get dehydrated on flights and it's nice to be able to sip on water occasionally while lying down.

edit: even in normal domestic F seats, a water bottle allows you to have water by your seat without having the tray table down and without having to worry about knocking a cup off of your armrest.