Old Microsoft: it was a BillG ethic: anyone touching software got an office: software design engineers, PM's, QA, even admins ... it allowed one to be quiet and focus. And signaled to co-workers - do not disturb ...
To contribute to the broader discussion; SOME teams here still have offices, but it's a dying breed. I consider myself very lucky that I found one that did, and it was certainly a factor in my choice.
This is second hand, could be wrong.