Part of it this was to develop a healthy rapport, to build mutual trust while working on large-scale projects.
But another aspect was productivity; both my team and the other teams had a lot on their plate, and it could be difficult to align incentives such that our shared projects would be completed in a reasonable amount of time. Flying out, finding a desk near the other team, making yourself visible/available, and working directly on the project would get more done in a week than would have happened in 2-3 months of remote collaboration.
Of course, it would be lovely if all companies could get work done remotely (yay Gitlab, Basecamp, etc. etc.), but most companies aren't set up for remote collaboration success, especially across management lines.