It's an outdated saying but still holds meaning. Like these days saying "send the horse to the glue factory". Way back when it was more common since glues were made from horse parts
Hide glue is amazing stuff! Smells a bit, but works super well for furniture builds when you want extra time to get everything clamped up and square and settled.
Earth society amongst itself was rid of a need for currency. However, other cultures/species within the galaxy (most notably the Ferengi), still operated under a system of exchange. Anything the Federation couldn't barter for, required some form of compensation. Credits, however they were denoted, were used to maintain the galactic economic flow. Even on DS9, Federation members were allotted a kind of stipend to be spent with the various merchants.