I didn't realize that Kaji was so experienced in regular gambling, to know such an obscure slots mechanic and something like the flower bet. It just clicked for me that this first chapter shows how Kaji really likes gambling. So Baku probably befriended Kaji not only because he was a good person, but because they both really like gambling, and Baku wanted to show him how to really gamble by taking him on his crazy journey to conquer Kakerou. Kinda like making friends of the same hobby, lol.