The currency helps everyone at the table keep track of who owes whom and makes it more transparent to the player what they are owed - which is something the characters in the setting will understand well, while the player may not.
Is all that necessary? No. But neither are combat rules longer than a paragraph or two of text using a single six-sided die for resolution.
No, see, I **LIKE** it when my PC's forget who they owe what. That's grist for the mill. I like it even better when they think the NPC's believe they're not owed "somehthing" for favors solicited(or not).
That pushes RP which pushes potential conflict/conspiracy which keeps the wheels moving. The non-stop negotiation of who owes who based on whatever codes of authority - Temporal or the Street - is the juice. I mean sure a PC could keep a Book of Grudges and notate all that (which would be a fun schtick), but to have it some kind of absolute currency seems silly.
"But the game is out there, and it’s either play or be played.” - Omar Little
For me the sauce is in players enforcing their will (or not) of their PC's on whatever the situation is. These are defining things that signal to the world they inhabit, who/what they're about. This is one of the most defining elements of self-agency in my game. Reducing it to a currency loses some of the flavor.
But I guess it depends where you like to play your games. I like mine up close where you smell the stink of fear.
I want to add - *I* most certainly play my NPC's with that same level of ignorance. My players can do things out of the kindness of their own heart without saying shit, but if my NPC's are of a certain mindset, they might look at those "favors" as exactly that - and act accordingly. They might even double-think it as if the PC's are trying to subtly threaten them - like "here I hooked you up. With the implication that "I can hook you down too, little bitch." and because they don't impress their actual motives on my NPC's - this leaves me to react accordingly, which all becomes notations for my NPC writeups. Which creates potential "good" or "bad" situations to emerge from those interactions.
And maybe it's because I grew up with Machiavellian motherfuckers among my family and friends. Heh.