It's still proprietary software. Why would I use this instead of Libreoffice?
Obviously there are reasons to use LibreOffice, like you said. But don't try to pretend there's no reason to use Google's office suite over LibreOffice.
https://wiki.documentfoundation.org/Using_LibreOffice_in_a_W...
For work: we use google spreadsheet for a lot of different status reporting. Allowing multiple people to edit it at once (and view all the changes immediately) can be really nice for certain use cases.
p.s. According to a sheet I have saved I can download my info as: .xlsx .ods (OpenDocument Format) .pdf .csv .txt .html
If you could export all your documents in the native format and import them into LibreOffice you'd still lose features, but as LibreOffice improved it's import you'd lose less.
With convert at export data is permanently lost. With convert at import, no data lost permanently lost, it just may not be currently available.
Also if you could backup in native format you could import into a google account if your first account is blocked/lost, instead you have to go through the import/export conversion twice.
Also, with Excel, once you have a computer running Windows / Excel, you can't lose data if a feature is changed / dropped. Just don't upgrade. With Google Docs, you have no choice to upgrade to the latest version. It may not even be a dropped feature, you spreadsheets may accidentally rely on a buggy feature, and when it's fixed your spreadsheets behave differently with no warning.