My thought would be that making those choices guarantees that the debt in question is a real thing in the fiction, that the rescued party has no choice but to acknowledge. Sure, the Angel can say 'you owe me' even without this choice, but who knows if anyone else agrees? If these options are chosen, then there is no doubt in anyone's mind that Rolfball was gonna die otherwise, no doubt about what that means for Rolfball and his friends, etc. You can't actually create debt/obligation unilaterally in the lawless post-apocalypse, after all -- not without some separate leverage, at which point it just looks like extortion with window-dressing. Choosing these options says 'this is leverage -- all by itself, this is a real thing.'
This makes it a particularly useful option to choose in a case where the person you just saved would otherwise be inclined to take your help for granted, or to simply dismiss your claims out of hand.
Now, there are ways to acknowledge a debt that don't involve paying it -- presumably lots of the people who live in the post-apocalypse are not big fans of reconciliation of any sort, and certainly not to their disadvantage. That's what makes the choice interesting -- or more interesting than just 'you get X barter from your patient.'