Yes, it is necessary. If you really want join that college. Please write it.
As the name says, they are optional, but I strongly recommend you write them, it is a additional chance to show the university who you are apart from your academic profile!
Just going to add my answer since the advice here is varying a bit. You should complete most optional essays, as they allow you to share more about yourself, and demonstrate your interest in the school. That said, if the optional essay is something like "explain an unusual circumstance," and you have nothing to explain, then you can leave it blank with no penalty.
Not necessarily, if you're looking to be admitted to a competitive school, I would say absolutely they are, but a college with a higher acceptance rate, I would say that as long as you had a reasonable high school profile, and especially if you have a strong academic profile.
If you want to stand out then yes. Its a chance to add to you narrative and personalize your application
Miriam Webster defines optional as "involving an option, not compulsory" (compulsory meaning necessary/needed) so its not required. Contrary to Yolanda's answer you should write essays only for schools that are reaches and targets and safety schools that have an elite program such as Honors that you want to get into. So that's my advice if you are a decent or better writer if you arent a good writer either don't do the optional essays or heavily edit them to make them great. Collegevines peer essay review is a great tool!
Please comment if you need clarification as Id happily help! :-)
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Please refer to my answer as this is 100% wrong and I suspect a troll.