It is a common thing to prove that he is the real owner of the address. Anyone can take any txid out there but it can be a valid proof that the txid belongs to him. In this case, above user tries to assure people that 1xbit is a legit site that pays him such a huge amount. With all other unsolved accusations against 1xbit, it is important for him to prove what he says. Signed a message is not a complicated thing to be done, so why not?