A tablet suits those needs a lot better than a $1,300 device that can do less locally than a $500 iPad.
She cannot do this with normal desktops (she doesn't know how to use file explorers).
You're underestimating how valuable it is to have your data be available anywhere, anytime, you're underestimating how much of a burden very basic desktop usage is for the average Joe out there. Sure, that's probably not as true for the newer generation, but lifting the burden of system administration (managing data) is a pretty significant deal.
Personally I hate the idea of anyone but me being the guardian of my data -- to me this is just an excellent method for Google to introduce their ads into everything. Even to access my own static data I will keep getting served with ads, I hate that -- but it looks more and more that this is how things will be in the world of tomorrow.
For the same reason a good chunk of my neighbors upgraded to iPhone5 from iPhone 4S (even though some of them didn't have the iPhone4S for more than a few months, and probably had to pay full (non-subsidized) amount to make the upgrade). Notice Google's marketing and branding efforts for this Pixel being eerily similar to Apple's, the pitch that the device somehow transcends in value over all of your other silicon-based products. They're trying very hard to have their brand be in the same rank Apple is in. It's a thing you're going to use to keep your pace in the 'Keeping up with the Joneses' game, it's like a Gucci bag, it's an alternative to the Macbook for when you're in Starbucks. There's no good reason to buy a Benz over a Corolla, but a lot of people do. Worse, a lot of people who probably shouldn't do.
If you really consider the actual market for computers, most users are going to laugh at Google's ridiculous price tag and buy a Windows machine for less than half the price.
I think what they're going for is the computer-on-cloud concept taken to new heights -- not worrying about viruses or malware (Google is effectively the new system administrator of your machine), not worrying about data being unavailable going from one device to another is the value proposition they're offering.
If you really consider what the bigger half of people use computers for (watching videos, social networking, basic word processing, etc.) it suits their needs well, and the value proposition will probably resonate with them.