IANAL, but I think it depends on what data they collect. Part of running any advertising business is accurately billing the advertisers. To do that, they need

IANAL, but I think it depends on what data they collect. Part of running any advertising business is accurately billing the advertisers. To do that, they need to measure enough information to track viewed impressions, click-through rate, etc. If they collect only enough information to perform business functions like these, I believe that would constitute a legal basis for processing under the GDPR. They may have a harder time using that justification for information that is only needed for ad targeting. In that case, consent may be an easier legal basis for them to establish, for which an account would be useful.