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> But, why?

For example you may want way to backup huge video files, panoramic photos or any other huge files. Obvious solution is your own storage, but usually you'll want at least one extra backup point because HDD these days a lot less reliable than cloud services.

> The upload time / cost must be enormous (on my home DSL it would take all year to upload.

It's sad that not everyone around the world have fast internet connection, but with 100Mb/s it's would take just about 10 days. I personally don't have that fast speed, but from my experience Microsoft OneDrive can handle like 70Mb/s upload just fine.