Any idea what the incentive is for them to put in an email address they can't access?

I run a few websites that accept an email address (all noncommercial, I have no interest in spamming anyone). One of them is the "contact me" feature on my personal website. To prevent spam, I had people just put in their email address and it'll automatically email them my email address. This works perfectly to this day, haven't got a single spam email on any of the addresses I've handed out, but the ratio of emails sent out to received is probably 50 to 1. Why would anyone put an email address in there if not to contact me? I've been wondering if it's used by mail bombing services, idk if that's a thing but I know of the concept of annoying someone by signing them up for a hundred newsletters. My site doesn't send recurring emails, though, and it doesn't allow putting more than two email addresses per month in, per /24 IPv4 block (and even more strict on v6). It's useless for mail bombing services but the (presumed) bots keep submitting a steady rate of maybe 2 new email addresses per day, each time from a new ISP in a random country. No email addresses is ever submitted twice. No rhyme or reason to it. If anyone can make sense of this, that might help me in stopping the abuse