Sibling comment nailed my feeling on this: the UI of onlyoffice is modern and clean. To add on, it gives me a feeling of confidence that it can round-trip files back and forth from excel more reliably; a lot of people on my team also use excel and we need to exchange workbooks, so that was what prompted me to move on from LibreOffice when I lost confidence that files would be able to remain pretty stable going back and forth.