Rakuten are the ones who make Kobo, a Kindle competitor that's more popular outside of the US - I have a Kobo.
Likely this is to eventually integrate into their Kobo device offerings, to let you upload your own .epubs (as opposed to Amazon .mobi). 10GB may be small fish for everything else but for ebook storage it's more than you'd ever need.
Tried it, immediately uninstalled on finding it needed a mandatory account created.