It's called competition. While a poker player does reap winnings from those he or she beats, no body was held at gunpoint. Every year in the WSOP, there is no guarantee. Some of the best players can be knocked out early if they get sat at a table with some rookie who has insane skill, and a little bit of luck to get better cards at crucial moments. It's a game of combined skill and luck. And you enter knowing you may not walk away with money.
They provide entertainment for many. They may not provide new services, but they help sustain current ones. For televised events, they draw in advertising revenue, and they also provide advertising for their sponsors. They provide/sustain employment for employees of host facilities/casinos. There are a few more that kind of go hand in hand with all of that.
Is it a necessary service? No. But neither is half of the 'professions' in the world. But everything is a variable and works off of the others. No profession is necessary unless its farming, delivery, market/retail, legal, civil, etc etc etc. But something new is created, and new professions open up, and those help open others... everything is a branch, and its hard to say there is nothing that isn't directly affected by another.