The state may have credits to do things off planet, but the average earth citizen wouldn't be able to buy food, clothes repair parts etc, because earth doesn't deal in money.
If hypothetically Earth did have a welfare state system without money, and the rest of the Federation Member worlds didn't, this could serve to isolate Humans on their home world. And if Human colonies also had a more conventional financial system like the rest of the worlds, Humans emigrating off Earth would arrive at their new home penny-less.
Still, emigration would offer Humans an opportunity to live somewhere that they could succeed and be compensated for their efforts, even if they arrived with nothing. Some of my ancestors traveled to Brazil as indentured servants, but it was better than what was behind them.
James T. Kirk: " We're on over a thousand worlds and spreading out."
Might be the reason there are seemingly so many Humans in Starfleet, it's a chance to get out.
Too bad trek simply didn't go further to explain how humans don't use money or that there might be some type of currency exchange.
It would not have really taken that much, if Gene Roddenberry was serious about it being in the show. According to writer Ron Moore, the writers did ask him about it on several occasions. Roddenberry simple couldn't explain it. Moore thought it was because Roddenberry himself could not himself conceive of how it would be structured.
Instead of Riker saying that he did not carry
money for a tip jar, when the piano player asked, Riker could have said something like money hasn't existed for centuries. (Which would make the tip jar on a Federation world kind of strange)
The writers could have had Quark complaining
about the inability to sell his shuttle for savage in Earth's system, because Hew-mons
long ago eliminated money.
The Neutral Zone would have been a excellent episode to have Picard tell the Businessman that the reason he's money was gone, is because everyone's money