Social currency with real world benefits seems the most likely thing to me. Look, if people could afford a Lamborghini with no payments if they just saved somebody's life, everyone would be all over the place looking for someone to save. That's the extreme end of it, but sure, give up a comfy life on basic income on a core world to help colonize, or join starfleet to help better society, you should get something extra for your time.
I think is the opposite. It is the people who get bored with driving around in their free Lamborghinis and the effortless ease of Earth who move to the frontiers or joint the Starfleet. Their motivations are not material at all, they want the (relative) hardships that such a life brings.
My best guess is that the Federation guarantees a basic "income" to Earth residents (food, housing, access to replicated materials) but they still respect the inheritance and distribution of legacy "wealth" (land, residences, possessions) and allow notable individuals access to "luxury" items as a reward for heroic deeds on behalf of society.
At some point, all the food and holodeck porn is not enough in your life. And for those who it is? Well, they stay in their house forever.
How can Raffi be poor on earth? In one of the episodes this season (I think it was the third one), Raffi is shown living a trailer, and her dialogue implies that she's embarrassed to be living like this. Whatever happened to no one on earth living in poverty?
Simple answer: She’s not. That space trailer is located in a stunningly scenic location, has power, a sonic shower, a fully functional replicator, uber fast WIFI, access to the entire Federation database. Raffi has access to free Trek-level healthcare, free travel throughout the Federation and no material needs. She may be “off the grid” but she is without material want. Her isolation is self-imposed, a result of her depression and addiction. And while she would greatly benefit by getting treatment for both of those things, to say she is poor is simply inaccurate.
She's embarassed to have lost her career, her status and her self esteem, not to mention for being a drug addict. Not because of where she lives.
It may be that you are viewing 'poverty' through a 2020 American lense. Do you say she is in poverty because her home isn't made of brick like Jean-Luc's?
That is NOT poverty by any metric. Poverty requires house, food and health insecurity. Not living in your ideal location is NOT poverty.
Did you grow up in a supposed "Utopia?" No. In The Trouble with Tribbles we see a station run by Humans that has a very obvious "American style" monetary system. Uhura was going to buy a Tribble prior to one being given to her. Uhura was on the station to do some shopping, Uhura had money. McCoy as in the process of buying passage on a ship in TSFS, in the same movie Kirk said "I'll hire a ship." In GEN Kirk spoke of selling a house. Picard made a purchase of a statue while on vacation on a planet that was part of the Federation. Quark, who was in Earth's star system, had to sell a damaged shuttle in order to pay for passage back to DS9. Yes some characters spoke of "no money," but there is repeated dialog that indeed the people in the Federation have money, financial institutions, commerce.
Nothing about her life looked like material poverty. She lived in a small house... but lived there alone (no kids, no spouse) on ALOT of land (whether her land or public land, we saw no other people, houses or things. She lived on the capital planet of the federation, which to the extent there is money, is likely harder/more expensive to do than living on a distant colony. She had massive (and likely free) access to civilization (taxi's + transporters + information + healthcare + food).If you assume a non-spaceship holodeck... imagine the possibilities of an outdoor holodeck or a holodeck room in a house, she did the equivalent of retiring from military service to a simple country life on an earth that almost certainly is incredibly urbanized. Now on the other hand, Picard is old money rich... having a massive winery in the suburbs of the federation's capital city (which has to be an incredibly unproductive use of land in general and specifically that land. He has the money to hire an armed black market ship with the spending cash to bribe Romulans who literally want him dead. This all being possible with what would be like a trip to the ATM for us (we didn't see him sell the land or make any effort to raise the cash for the trip.
I'm not even sure that having a vineyard makes you 'rich.' Most food is replicated, I really don't think there is a shortage of land. Perhaps anyone who wants can have their own hobby farm or vineyard for free. It's just most people don't bother with such.
If most food is replicated (and most alcohol synthetic), that would make the real thing all the more valuable. People in the TNG era were talking about how replicated food didn't live up to the real thing all the time.
I don't think that most of the time people would care that much. Besides, if there's no money you cant sell those things, you can just give them away for free. And I assume that this is exactly what they do. Yes, you can get a vineyard, yes, you can make excellent wine, but there is no monetary reward for it. The reward is people complementing your wine.