In the past, didn’t sounding rockets punch right through the balloon? No snags?

To add more detail to john hare‘s comment:
1. The balloon gives the advantage of launching from any latitude, which allows for an eastward launch for maximum efficiency as little or no energy imparted to velocity is wasted on a northern or southern trajectory. With an eastward launch, the latitude translates directly into the orbital inclination to the equator.

2. Due to the high starting altitude, max Q is a lower force, allowing for lighter fairings and more energy imparted into velocity.

3. Due to the high starting altitude, air density is significantly lower, reducing energy losses from aerodynamic drag and a reduced delta-v expenditure due to this drag.

4. Due to the high starting altitude, horizontal acceleration can begin right away, reducing the gravity drag by perhaps a minute, again reducing the delta-v expenditure.

5. Due to the high starting altitude, vacuum engines can be used, giving a better specific impulse (Isp), which is a higher efficiency use of the propellants (more delta-v per pound of propellant.

6. Around 10% of the energy expenditure is altitude, so a fourteen percent reduction of altitude (350 km orbit) is another 1½% reduction in delta-v expenditure.

It all adds up to greater payload capacity or higher orbital insertion.

A launch from an aircraft, e.g. Pegasus or Virgin Orbit, gives about 600 miles per hour of initial velocity, which is about 3½% of orbital velocity. The aircraft cannot get as high as this balloon did, so the other advantages of the balloon launch (2 through 6) are reduced. Do these two methods break even? I don’t know, but I suspect that the balloon is less expensive than the aircraft, so there is also a price advantage. Oh:

7. Reduced launch costs and less ground support equipment — which would necessarily be portable and storable offsite.

8. You don’t have to worry that your proposed launch base and its pads will be voted down by the neighbors, as happened in Camden County a couple of years ago. Similar problems in Scotland.

Launching from a balloon at altitude gets you the ability to use a vac nozzle for higher Isp. Elimination of most of the drag losses. And some ability to thrust horizontally earlier. That is several hundred meters per second savings applied to the rocket equation. If the balloons didn’t come with as many issues of their own I would expect the technique to be more common.