My sense is that if writing was entirely optional, it would be too easy for students to let it slide; having 10% count toward your grade is a good "nudge" to get honest students to actually do the work. I'd just give a little admonition at the beginning saying that I'm not going to bother checking if you use LLMs, but if you do you're an idiot, because you'll be completely unprepared for the hand-written essay you have to compose yourself in the exams.