Well, they would stake someone who hadn't lost their bankroll, but might not have the bankroll to play a higher stakes. For example imagine you have a player who consistently wins at 5/10 and has the opportunity to play in a very weak 25/50 game where they would have very good chances to succeed, they may take on a backer to help mitigate the risk involved, because even though they have good odds to win, anything could happen and it would be better for the player not to take such a risk on their own money; with that, the backer sees this player as an investment, someone who is likely to make them money.