I use Proton Drive as my main data storage just for convenience. PDF files to read, little projects, not critical administrative stuff, etc. If my smartphone is stolen for example I still have my data.
It highly depends on you. For each kind of data, should it be stored on a hardware you own (computer, smartphone, USB key, ...), on cloud (Proton Drive, Bank, ...), on paper (book in library, paper in safe box, whiteboard, small card on your pocket, ...), on your memory, ...?
The data storage problem is a wide problem both for individuals and companies. Difficult to make a clear answer that will perfectly fit your needs.