The point is to go to a foreign league where the players are true professionals but at the same time not that good where they can take your shine away or arouse doubt in your potential. Heck even going to a good team and getting glued to the bench might be a good enough opportunity. Less risk of an injury and you get to learn in practice against fully developed pros and see how actual sports organizations are ran.
Jennings was the first prospect from the YouTube hype era to do this. He got a nice bag 1.65 mill from Roma and 2 mill from UA. He didn't play particularly well and didn't play alot of minutes. Euroleague teams are notorious for not playing young players and making them earn their dues. Despite this Jennings showed he was very well prepared for the NBA in his rookie season.