I think it's for humans. Often humans looking through SQL code are doing so for performance reasons, and a recursive query is fundamentally different from a non-recursive query. It's something you might want to know while skimming, before diving deep.
I see where you are coming from but the idea does not resonate with me. A CTE is a CTE, recursive or not. Sometimes you do not care as long as it generates correct data.
I can see an analogue though: some languages differ Sub from Function although they are basically the same idea.
On the other hand, Postgres seems to be the only RDBMS that makes the RECURSIVE keyword mandatory for recursive CTEs.
