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.
When you gamble for a long time and can predict the ratio of win and loss which will be very accurate then you will consider yourself an experienced gambler. Although no one can claim to be an expert in gambling because luck plays the most role here. However, our emotions often make us think of ourselves as experts. This is only the effect of our emotions and greed.
Since it is not possible to guarantee who will win and who will lose in gambling, many people make predictions and increase or decrease the amount of their bets, and if they are lucky enough to bet on a large amount, they get a very large return. And then they start thinking that they are very clever. This is always seen among gamblers.