These sorts of units have some utility.

A year back I was in a tire place waiting for them to slap new shoes on my truck and a lady comes in and asks the guy manning the desk if she could get a jump. Now this place was mostly a tire store with minimal capacity for repair. A good place to get compressed air for tires but not the place I would think of getting a jump. I was just about to offer to help, they hadn't pulled my truck into the shop yet, but the man at the counted said he would help, So he leans into the garage area and grabs one of these battery units and follows her out the front door. He reappears after a few minutes.

After A bit I ask him about it and he says that it happens all the time. Somehow people just figure that if you work on cars you are set up to jump-start one. The owner of the business, working the angle that doing people the favor of getting their car started was good advertising, arranges to keep a couple of the units charged and ready to go.

I thought they might be special commercial units but when I went into the work space what I saw was the same units you can buy at any discount store. They were the top of the line for discount store units but they weren't anything special. One of the techs noted that the consumer grade jump battery units work pretty well as long as you keep them plugged in and charged. Most people needing a jump just need a little extra power and there gel-cell battery units are light enough to easily carry around a large mall parking lot.

After this I took to looking around all the garages I visit and have noted that a lot of automotive places, even parts stores, seem to have one of those units. I'm not sure they make sense for an individual home where, assuming the vehicle/s are well maintained, the times you might need it are few and far between. Particularly because the gel-cell will deteriorate over time.

An alternative might be to get an inexpensive battery charger. With the newer pulse technology electronics these units have dropped in price, size and bulk. Used to be even a small battery charger was a substantial hunk of equipment that was also a substantial investment.

The newer units are cheap and light. They also seem to work well and last so for a small price it makes sense to have one on hand.
Larger chargers have a 'jump-start' mode that will get you going fast but even smaller units will charge a mid-sized battery in an hour or two. The weakness is you need line power to run them. But if you know an emergency is coming, like the days before a hurricane, there is no reason you couldn't charge up all the batteries, possibly even a reserve or two, and have them all GTG when the time comes.