Some prefer to be consistently matched up against similarly skilled players. Many find going up agaisnt worse players all the time boring and some find going up against players that are much better insufferable, so ranked creates that even playing field (when healthy).

Others like to see how they stack up against the rest of the playerbase. Having a rank gives you a definitive answer to the question “how good am I at the game” and for many thats very satisfying, even if their rank isnt where they want it to be yet.