If half the team is in office and half is remote on Slack, then the communication throughout (both useful and pointless) is much higher among the in person. Some simple examples:

1) quick clarification for a technical question - easier to turn back and ask your colleague

2) design minutiae which you remember after a 30/60min meeting is over - very easy to grab your coworkers in hallways or by watercooler and nail down

3) coming across interesting conversations among your colleagues by just being in proximity - not possible when you have to schedule a 30min chat for that

4) reading body language to know whether your colleague is stressed or relaxed or happy or sad - helps tailor your response which improves the communication

I haven't even delved into the timezone issue yet, which matters a lot even between colleagues on East vs West coast (let alone Europe or Asia). Meetings before 8:30am PT? PT people would still be waking up or dropping kids to school. 9-10am PT? Time for people in ET to have their lunch. 12-1pm PT? Lunch time for PT people. After 2:30pm PT? ET guys are already out for the day.