I think in larger companies it becomes even more critical for a PM to shield the development team from ad-hoc sales/marketing requests. It's not that anyone is ill intentioned, but everyone tends to think the problem in front of them is the most critical, and may not be aware of everything else that's going on. A PM will hear any request, know the context of the request as well as the context of what the dev team is working on, and manage the expectations of the stakeholders if necessary. In a small startup, I would actually expect that this part of Product Management would be less important, since it's more likely that stakeholders are more aware of everything going on. In large companies, I think it would be crucial for keeping the development team happy & productive.