To me, sportsbetting is entertainment. If I'm in Vegas and I'm up a lot of money I would bet $100 on a team and if they win, I consider my next trip to Vegas "free" since the $200 would cover a lot of my expenses. Or if I'm watching a game that I don't care about which team wins, I would place a small bet on one team and root for it. Even a measly 10 cent bet on centsports.com would make me excited about a game I otherwise wouldn't care for.
There are some people that make a living sportsbetting. All they need to do is beat the vig consistently by picking more than 52.4% correctly in the long term. By following a few teams closely, they can handicap better than the people that set the lines. Someone who picks 55% correctly would have a 2.6% advantage in the long run, better than card counters.
There's also arbitrage of covering every outcome with a positive result. That's done using at least 2 different sportsbooks. For example, a sportsbook in Europe may not know much about NFL football so their odds would be different than Vegas books. Also, if an international game is taking place, sportsbooks in the participating countries would have heavy betting on the home team. The point spread wouldn't accurately reflect the expected outcome, so arbitrage is possible.
