This chosen figure is not only its value but also its percentage in your general finance and regular income. $100 can be small with you but all money another person has for example so if $1,000 is small with you, you can use it for gambling very feasibly and comfortably. With the other gamblers, if their finance is not good, they must spend a much smaller fund for gambling as $100, $200 or any gambling budget most affordable with them.
It's is not only an attemp to minimize your loss from gambling but also to avoid unnecessary dramatical effects on your finance and life quality.
I am a player for whom a figure of $100 is quite a lot and in that case I would divide it up to 2 weeks and each day I would play with little money, financial health is extremely important to me, because I only use a large part of this income to buy BTC for a very long time applying the DCA method, but for the game I use little money, I have had bad experiences spending a lot of money, that is why now my thinking is different.