Nothing is really. It has its uses but it's incredibly situational. There are some Pokemon that make good use of it though, one example being Hitmonlee who uses a combination of Endure, Liechi Berry, Unburden and Reversal to get itself a boost in Attack, an overpowered STAB attack and a big Speed boost that makes it difficult to revenge kill.
Basically it's only competitive use is to make yourself survive your opponent's attack, which will hopefully knock you down into a range where your ability or item will activate (in that Hitmonlee example it's the Liechi Berry which gives an Attack boost if you fall below a quarter of your total HP). It isn't used to stall, Protect does a much better job at that.
Other than that it's just one of those annoying moves like Protect that NPCs in the game use to be annoying and waste time.
