Virtualized personal assistant services aren't really anything new. This one just had an interesting/novel/awesome (and probably unsustainable) approach to the onboarding, UX, and price model - instead of "hiring" a "virtual assistant" and doing a bunch of sign up forms and paperwork, you just punch an address and card and away you go.
As for why people want a personal assistant to shop for them when they could just use an app, think about it this way:
To get things (even online), I have to:
* Decide what *specifically* I want (brand, item, size, etc.)
* Decide where to get it from
* Obtain their website or app
* (often) Sign up for an account
* Enter a bunch of information to order what I want
* Follow up if it's late or wrong
With this service (or a hired personal assistant) I have to:
* Decide what type of thing I want
* Tell someone to make it happen
Also, I wouldn't call Instacart or Expedia's interface "perfected." Thanks to the way the industry works there's still not a single website where I can say "find me the cheapest flights from point A to point B in date range X" and have it
actually do so. With Instacart, I can't just punch in a natural-language grocery list and say "get me decent quality options for each, but make the beef grass-fed please." I have to go through a huge array of brands and potential sources for each item and decide what to get.