I would have to assume that credits are a largely bureaucratic unit of account that most Federation citizens will never work with or even hear about, but are used by internal departments of the Federation as a means of budgeting the capacity of things like transport ships, industrial replication facilities and shipyards.

This also allows them to function as a de facto currency for trade with outside powers who have achieved warp travel but aren't yet in a post-scarcity state, or as a way of managing resource usage at the edges of Federation space where infrastructure is still developing and resources need to be priorities for that development.

Basically, the Federation doesn't have "money" as an everyday societal phenomenon, but it does retain the economic capacity to issue something usable as currency when the situation calls for it such as during periods of scarcity (whether localised or across the Federation) or when conducting trade with non-Federation entities such as the Ferengi.