Grocery shopping is literally the only reason I still own a car and pay ridiculous city insurance rates. I don't use it to go to work, meet friends for dinner/drinks, or anything else for that matter.
And then of course, there are times when I couldn't be arsed to go grocery shopping :)
Tomorrow, I need to go to Ikea to get some other items, since I wasn't a fan of what's available on Amazon, or the pricing on other sites. I plan to use ZipCar.
Instacart is one of the many services that are helping me to lead a car-free lifestyle here in Palo Alto. I'm from Toronto, and I wish I could have accomplished the same when I was there.
I do the above in SF.
Even with grocery stores nearby, you still have to go fight crowds of people, take all the time to find everything on your list, wait in line, wait to get checked out, then get your stuff and walk back to your place. A single trip can be exhausting, and even more so if you're introverted and/or hate dealing with crowds of people. Narrow aisles, the elderly, and annoying loud/crying kids can make for an infuriating trip if you're just trying to get your stuff and go asap.
Or... you could use something like this and have all of this taken care of for you.
Grocery stores are fairly abundant in cities. Within a four-block radius from my house, there is a Jewel, a Whole Foods, four corner stores, and a Trader Joe's under construction. I honestly don't know how Instacart maintains business in San Francisco proper.