The difference here is though that there's nothing of value.

If I wanted to use this for malicious intent, then I think the most valuable thing I could get away with would be a 5% discount coupon.

I like that you bring this stuff up though as it could perhaps be good to mention this on the site down at the FAQ so people don't think I'm doing something shady with the service since there are no ads.

Or spinning up hundreds of accounts on a website so you can perform card testing. Or creating accounts so you can put all of the inventory for an e-commerce site into carts to reserve it for yourself. Or running sneakerbots. Or any other malicious intent where the victim is the website operator.

In this day and age I don't understand why they wouldn't be able to just use a headless browser instead of doing it manually - has labor really become this cheap?

You can get all the stuff you mentioned for a fraction of the time and money it would cost to create a disposable email service and then you would also only target the service you wanted instead of sitting around and hoping that a user creates an account at the site you're looking for.

The real value would in the reputation and history of the account (e.g a popular account on a social media site or community) and this isn't a thing that someone would use a disposable email service for as they wouldn't want to risk losing access to the account.

But as a developer of these sites I've used the majority to check my own domains and the most reliable one I've found is https://www.ipqualityscore.com/

I strongly recommend that you take another look at your signup process though and don't require users to give up their email addresses, instead make it optional and present an alternative like a captcha.

Feel free to share a link to your service as I'd love to check it out and best of luck running it also!

A separate use case is when there's a free download that's email-gated, rather than account-gated. Examples would be Gumroad, certain 3D print platforms and such. I'd rather not wind up having my email just be added to some marketing list.