I don't have all my juniors around me, so I can't overhear their conversations, so I can't jump in when they talk themselves into trouble. I can't help people debug problems I'm not aware of, and frustrated grumbling often comes a few steps before actively asking for help. I don't have people from other teams around me, so I can't grab them for coffee/lunch and catch up on what they're doing. I used to have a couple of PMs from other teams who regularly came to me for help because their engineers weren't great at talking to non-technical people.
Fundamentally, information is the biggest asset you can have on the job, and I lost access to all the information that came from organic, unstructured, ad-hoc, and often passive interactions. Unfortunately, the more structured, proactive alternatives cost a lot more of my time and effort, so it's all around less effective.