I have forms on my in-development website which require the person to enter an e-mail address.
I don't want to have to send them a "verification" e-mail with a URL containing a code, or instructions such as "please reply to this e-mail to verify that you sent it". This is extremely likely to cause problems such as them not receiving it, or not seeing it, or forgetting about it, or don't want to click on URLs for whatever reason. I know that it's "best practice" according to numerous online guides for e-mail, but in reality, it's a major chore for everyone involved.
And having a text such as:
If you wish, you can click this link to verify that you sent this: blablabla
feels pointless since most will not bother unless they have to.
If somebody enters a nasty message into the form and states their enemy's e-mail address, my system will therefore assume that the person owning that e-mail address submitted the form, and perhaps I will add him to my "blackhole list" without considering that it may not be them. And then, when they later actually want to use the service, they can't, because I added them to my list of e-mail addresses to silently ignore, because some entirely different person "framed" them.
Maybe this doesn't happen often in practice, but I've had it happen to me personally, so I know that it does happen, at least sometimes. I've received angry e-mails from people replying to me after somebody wrote something mean and used my e-mail address.