It won't work offline, for one. I use Office exclusively without internet as otherwise who knows what private stuff it's sending back to Microsoft, which then becomes eligible for warrantless surveillance by the federal police.
I know it's old fashioned to want local software to only change things on my local file system, but if I didn't want that paradigm, I'd just use Google Docs.
Yes, this is important where you're online but not on a high throughput connection or any part of the connection has intermittent faults.
The disruption from even a few outages is enough to prefer to run things locally. We had issues with people unable to sign into Excel and everyone went mad (rightly so, as Excel was running locally not 365 but it blocked usage)
And on top of that, there are scenarios where you'd want to process substantial amounts of data that would be prohibitive to upload, process and return the results, where locally handling it would take way less time.
I eventually gave up on it because of the locals vs cloud differences. The documentation for the whole thing was terrible as well. I think they’re in transition (assuming cloud only). In my case, I just went and used Calc from LibreOffice.
I know it's old fashioned to want local software to only change things on my local file system, but if I didn't want that paradigm, I'd just use Google Docs.