It's annoying that these "hallway conversations" became the thing to highlight as the main advantage of in-office work. It's not my experience that the random encounters next to the watercooler or the bathroom are often that useful.

But, say you have some half-baked idea and want to talk it out with a coworker next to you. You both roll over to the whiteboard in your pod and start scribbling, both of you exchanging high-bandwidth information via utterances, gestures, and facial expressions with millisecond roundtrips. Third teammate overhears and rolls over, contributing further. This scenario IME used to happen a lot, created a lot of value, and is hard to replicate with a scheduled meeting or documents because the communication roundtrips and bandwidths are atrocious.