Pull Requests are great for understanding the larger thinking around a change or set of changes made to a repo. Reading pull requests are a great way to quickly “grok” a project as, instead of small atomic changes to the source, you get larger groupings of logical changes. Analogous to organizing the lines in your code into related “stanzas” to make it easier to read.
I find myself looking at a file or a commit, and I wonder if there is a way to backtrack the commit to the Pull Request that originally created it. That Pull Request would have been merged eventually, but not necessary with a merge-commit.