Even if you don't get essays marked, writing essays reinforces the knowledge of texts and you become a better writer. If I wanted to learn how to play tennis, true, it would be best to have a coach critiquing me, but nobody can deny that actually getting out on a court and hitting tennis balls will help me improve.
Not really, if you are hitting tennis balls with the wrong technique, really, you're only digging yourself bigger problems by becoming adept in this wrong technique, which only makes it harder to rectify as you have to train yourself out of bad habits that you have formed. Similarly, no one is going to have perfect knowledge of a text or how to write an essay and they can always improve. However, if someone keeps structuring their essay wrong, using irrelevant information or has such technical problems such as not addressing the prompt, what's the point?? It's only drilling in bad habits that will take even longer to fix than waiting for your teacher to correct your previous essay.
It's much better to keep a journal and write for 10-15 minutes each night about what you went over in class, write down a quote and analyse its meaning or describe a character's mindset in a scene than to aimlessly spend hours writing full essays that are just making the same mistakes over and over again.