A Volt can never be plugged in and operate 100% on the range extender (ICE).
A Volt can be run as an EV using as little as 3/10 of a gallon of gas per year (just enough for Engine Maintenance Mode, to ensure the engine is run just enough to be reliable and for the oil and fluids to be circulated periodically (.03 gallons every 6 weeks does this).
A Volt can be something in between.
It isn't economical nor does it make sense operating in gas-only mode without plugging in.
It doesn't make much sense as a very heavy EV only, due to low range and still having some iCE maintenance to deal with.
It's really only good as an in-between car if you're ignoring things like the smooth electric drive train. And for most commuters (those between the 40-80 mile daily range) get much more savings out of a cheap Leaf or smart ED lease and renting or just using the household's second car for rare longer trips than they would buying or leasing a Volt. It's definitely not a bad second car though for people who can't yet afford a Tesla as a second vehicle, if I had room for two cars in my garage I'd talk my partner into a used Volt over a new Prius as her upcoming car--however, it still comes at quite a big premium over something like a Prius C due to the huge initial price difference.
When I was shopping, a Volt lease was over $100 more a month than a Leaf lease, which in itself is $100 more than a smart ED lease.