If you ever get the opportunity to play slots games on a site without any specific requirements, then it is very beneficial. Because slots games are operated by machines, a possible prediction can be made by placing very small bets for a long time and observing the winning rolling of the machine. Although everything in gambling is based on predictions, you can never expect anything big from it with a guarantee. If you are lucky, of course anyone can win something big at any time. And gambling companies constantly give very big reward winnings and only those who have very good luck get them. Being successful in slot games is not easy, but those who succeed get something big.
I agree with you that there is no guarantee in gambling, because it will only depend on luck. The problem is we never know when we will get such great luck, so all we do is try and try to be able to get lucky and succeed in getting a very big win.
I also don't know what the definition of a professional player is, are they those who never stop playing? do they know when they will get lucky? (and honestly I'm not sure about that), or those who have continuous luck? also I'm not sure there are people who continuously get great luck.
In my opinion, professional gamblers or professional players are those people who study gambling a lot and start to see gambling as a source of income. They spend a lot of time studying the games and have good bankroll management. Also, in most cases, these guys write books, have telegram channels and give courses. Now, whether or not they have long-term profits, I honestly don't know. They probably also have a lot of losses, which is why they create paid telegram channels to give courses.