Do you think it is necessary to give tips or it is just a tradition that is required?
It is a tradition, but it is not necessary depending on where you are and the amount you make.
The tipping culture that America has is totally bizarre and alien to how most countries operate, so it's a bad guide stick to how you should be expected to react. Why would you give anything specifically to "slot machine attendants"? It's not like they're a croupier that is running a game table which requires a lot of concentration and skill to perform. These people should be paid an adequate wage to do a rather basic job, which I can only assume entails helping customers with how to play games and maybe running drinks - hardly an intense job or something special. Sure, if you get a big win you might tip a staff member or two who are close by, besides that I doubt they are expecting anything more.
Tipping in US casinos is like tipping in restaurants there. There is a more widespread culture about it than in other countries in the world, plus the base salary for waiters is low and they supplement it well with tips, in return giving you better service. I'm not 100% sure if it's the same situation in casinos but I understand that it's the most likely scenario.
If you feel to give any small amount in return f their good service, that is not a problem either.
I prefer to give them in this case, feeling that they are providing me with an excellent service, not just someone doing their job.