Agility is a crew skill available to only crew positions in ground vehicles.
The level of agility determines the speed with which a crew member can replace another fallen one. The average value of agility in the crew is what counts - spending all of the points on one disposable crew member won't benefit the player any more than distributing the points would.
The replacement of crew is done following a priority - tank driver and gunner are the ones that are always replaced when taken out, whilst commander are machine gunner are less important positions in War Thunder. The loader position lies in between in regards to priority, since its job can be done by the man in the gunner seat, but is nevertheless an essential one.