when playing a major Event especially overseas and there are commentators over a tv or radio network they normally sit at the side netting and so can only see the sides of players as usually the players name is on their backs, and in front of a commentator there`s a sheet with players names against their numbers, they would look like plonkers if they kept saying there`s that bloke over there and another over there, so that`s a good reason to have a number on your arms.
however , over here in the UK when playing in general then NO it`s not necessary.