Oh, boy! As an American consumer, I'm even more perplexed what the hell they are.
Like 15 years ago, Rakuten seemed to be a normal ecommerce site. I think they bought buy.com or something to get a foothold in the US market. Then they pivoted to being some sort of cashback referral service.
I'm not really sure why that would lead customers to think "yeah, I want cloud storage from the people who made a weird janky digital simulation of the Piggly Wiggly Value Club Card!"
(AWS made it work because they could say "we have the infrastructure to host one of the busiest sites on earth, it's good enough for you", but Rakuten does not have that credibility in the US)
They do a lot of things, mostly related to online retailing in Japan. They bought Ebates, presumably to get all the shopping data they collect, it wasn't really a pivot. (Also they'd been doing financial-ish stuff like rewards programs before apparently. I went and read their Wikipedia article when I saw this post.)
It is common for SaaSS dis-services to charge a monthly fee for use. Usually one SaaSS site does not substitute for another, so if users become unhappy with one dis-service provider it is no easy matter to switch to another. When users become dependent on one, it can gouge them at will with repeated small price increases that over time add up to a lot. We view the loss of freedom inherent in SaaSS as worse than the cost in money, but when a dis-service has you over a barrel, the cost can be painful. Thus, even users who don't see deeper than the bottom line should beware of SaaSS.
People aren't storing massive amounts of data on cloud storage. For text document storage or even a moderate number of images, 10 GB is enough for many people.
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.