In OSS, after merging a PR, generally you can't get back at the author and make them do a follow-up change, because they are volunteers. A certain degree of gatekeeping is necessary. In a company however, you absolutely can just assign such a task to the author or someone else. That means it's possible to integrate changes into the main branch faster, which is very beneficial.

I've seen teams that try to mimic OSS development and only merge absolutely perfect PRs, dragging out long-living branches and integration. At some point the perfectness can't keep up with the plan, huge PRs are suddenly reviewed hastily, and the end result is worse and slower.

Bottomline: PRs can be used slightly differently in companies to improve speed and reduce overhead.