Gossip and social interaction.
It can seem pointless but when you find out that the random co worker shares an interest/friendship with a senior figure in the company, as you witness a brief exchange. A major mistake many make is not realising who "knows of" others in the workplace, they may normally never even greet one another.
The mail/parcel facilities guy mentions he is helping set up that new set of desks or has to come in early for a major delivery. Small things but can be very insightful.
On nodding terms with the help desk person who then happier to help you when you need urgent help.
Making polite small talk waiting for a meeting or con call to start. About some minor incident, feature or event in the office you both use. Even just waffling on about the carparking situation or the local supermarket or sandwich shop you all end up at when visiting the Office.
When you send that email to the team and whoever else. But you get that brief bit of feedback (it might just be a glance and a grin) across the desk that you just won't get via remote messaging services.
Overhearing or partaking in random conversations in the office which a snippet of become useful sometimes months or years later.
Having impromptu training sessions as either as teacher or pupil because well we are all there and now is as good a time as ever.
Opportunities as the person speaking to the person who is first asked.
I got offered an internal extra role which involves little but ensures I get an additional payment monthly on top of my wages and useful on the CV. All because stood by my then managers desk . I have changed my main role several times in the 9 years or so since but still have this little extra role and the payment.
Hosting and receiving guests on behalf of others or the department as in the Office.
Over the years I have made a lot of contacts this way and got involved in a number of projects and ongoing operations simply from being the stand in host or dragged along by a manager/director as their person who understands technology or simply for a second opinion..
Plenty of times sat in the office and become the resolver (even if asking those working remotely) in the eyes of those that need help. Even though credit given to others the person helped remembers your face and name.
Getting offered the extra tickets for a event the company sponsor as others pull out and well your there.
As I look back over decades of work it's nearly always meeting people in person even in passing where things happen. A quick interaction which I have forgotten may be remembered by someone who comes back into your working life decades later. Much less likely your recall those you once called or emailed in passing years later.
I work at home regularly but would be extremely concerned if unable to regularly visit my employers offices.
Never be in the office to often but been there regularly enough.