The trade-off is that you have to specify it almost every single time, but it lets you group form elements that for UX purposes belong together, but for other reasons need different forms.
[0] https://developer.mozilla.org/en-US/docs/Web/HTML/Element/in...
I had a project that had applications associated with it. Forms are the natural way to express this. There were some disclosures associated with the application, and there was a modal associated with emailing the disclosures. I needed to add some front end validations to this modal.
No matter what I tried, I could not get these validations to trigger correctly. I spent an entire work day on trying to get a library to work with this modal. It wouldn't work, because changing the modal's div to a form tag would cause errors or unexpected behavior due to the nested form tags.
TL;DR: Nothing requires it. Devs might not understand the implications.