Uncontacted tribes are not the only ones that have used such systems; plenty of other societies throughout time have used similar systems, some quite recently even. It is not antithetical to modernity. For a recent example of a society that used a gift economy, you can look up “Korean People’s Association in Manchuria”. I was using uncontacted tribes merely as one example to illustrate that the idea that bartering and capitalism are “natural” and “how it always worked” isn’t true, despite that being what many believe.
It’s ultimately more efficient to give people money and then they can spend it on what they need or want.
Why is it more efficient, exactly? In a gift economy, you don’t have to give anyone money for anything and won’t starve for not having enough money. In a gift economy, you help each other where possible and do things such as art or science for fulfilment and not because you have to put food on the table. Someone who can help, but rarely does, slowly begins to get shunned by the rest of society.
EDIT:
To read more on gift economies and anarchism in general, you can read:
Petyr Kropotkin’s Conquest of Bread is a good one; that’s more theory
George Orwell’s Homage to Catalonia; a sort of memoir of Orwell’s time in Catalonia fighting alongside anarchists
Ursula K. Le Guin’s The Dispossessed; a sci-fi story about a futuristic anarchist society living on a planet that mutually orbits another planet that is inhabited by other societies.