Never underestimate the kind of mess lawyers can think up. I know of a big organisation providing all kinds of services. I was involved from the sidelines in GDPRing a small part of a very unimportant and almost forgotten service of them.
My personal guess was they wouldn't need consent, as it was a clear-cut case where all requested data was clearly needed to provide the service. And asking for consent to use the data a customer just typed in the site for requesting the service seems a sure-fire way to annoy them without any upside.
Then the GDPR lawyers came.
It turns out, if you interpret the company charter in a very nasty but still legal way, there might be a very rare edge cases where a service was provided to someone who was not strictly 100% a customer. Yeah, I'm a bit vague here, sorry about that.
To be clear: No real-life example was found now or in the 10+ year history records of the service, both the almost-but-not-quite customer and the company would have to do insane things, and service delivered in the case was almost non-existant, but theoretically, on paper, it was possible. I'm pretty sure nobody would care if it happened, either.
So boom goes our legal base. Consent it is,then.