This is actually a fairly retro device as reflected in the circuitry. Its original purpose was to test neon signs in situ to find faulty tubes. In a traditional neon sign there can be several sections of tubing in series on a transformer, and if one loses vacuum internally it effectively goes open circuit.
This unit is basically a high frequency and high voltage transmitter. When you touch the antenna to the side or end of a tube it will cause it to glow if it is still operational.
They were also sold in electrical retailers for testing fluorescent lamps, but weren't really suited to that task as they do not show the state of the electrodes, and only give an indication if the tube has a seal failure, which is rare.

The circuitry is interesting though, as it has strong similarities to a device called a jammer that was used on some early electronic equipment. By inducing voltage in circuitry or wiring remotely it can cause unexpected results, and in the past similar devices were used to defraud petrol pumps and gaming equipment by interfering with their operation.
Modern cash equipment has extra protection to detect things like this to allow them to shut down and trigger an alarm or in earlier equipment just force a reset.

An interesting recent example of the electrical interference was when a sign installer was driving his truck to a job and his helper was playing with the device. The helper touched the antenna to the truck door and the engine management system cut out forcing the driver to make an unplanned stop on a layby. When they turned the ignition off and on again it resumed normal operation.

These devices have other uses, but their primary use is in the neon sign industry.

Be aware that these units can damage adjacent electronics without contact. Use with care.

Here's a typical search link:
https://www.ebay.com/sch/i.html?_from...
Target price is less than $20 shipped.

If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:
http://www.bigclive.com/coffee.htm
This also keeps the channel independent of YouTube's advertising algorithms allowing it to be a bit more dangerous and naughty.