Basically, it's for the entertainment value:
So why do I still buy lottery tickets? Definitely not for the expected monetary return on investment. I think of it as a discretionary entertainment spend. I get literally hours of enjoyment from fantasizing what I'd do if I won. I happily spend $25 for two hours of entertainment at the movies, and I don't judge the value of that experience based on its expected return. For me, a lottery ticket for the occasional big draw has just as much entertainment value, or more, than the many other things that I spend money on to entertain myself.
The decision of whether to buy a lottery ticket shouldn't be based on the probability of winning, or the expected return of a ticket, but on the entertainment value that comes from imagining a different life. If that entertainment value compares favourably with other activities with a similar price, then go for it. Plus, it has the added bonus that you might actually win; one-in-a-million events happen every day. Someone eventually wins the big prize, and you have to be in to win.