Entertainment is the correct reply in my sense. People are different I know that. Someone definitely think that making donations are much better and worthyer. But I'm not that type. I gamble to spend my free time on a nice wave, to take a risk and have some fun. To gain some profit from that also would be nice 
Maybe some people search for entertainment in gambling, but not all people playing gambling because of entertainment. Other people will reason for playing gambling because of making money. But if they know that playing gambling cannot make money, they will not use gambling as a source of income. We have the same reason for playing gambling for fun and use free time to playing gambling.