Being the same place.
In business, co-located businesses can have both synergy and competition, depending on the nature of the businesses.
Co-location between people is a strong signature of acquaintance or friendship. You can accurately infer social ties from colocation: Crandall2010inferring.
Colocation also induces more communication between people. When we meet someone we know in a place, we are likely to talk and follow up.
COVID 19 pandemic limited mobility of people and thus reduced such encounters. Carmody2022effect measures the impactd in research ties.