I'm glad you asked! (This will definitely be featured in the handout.)
Most people when they etch use sodium chloride solutions. Will that work? Short answer, yes. However, not only do people fail to properly decontaminate their metal first,(kudos to this guy using acetone!) but your electrolyte is going to become a toxic soup after a while. Not to mention it offgases chlorine. You could get around this by being well ventilated? But is that what you want?
Using an appropriate metal salt not only eliminates the gas issue, but if your metal is clean and ONLY the metal you want to etch is present in the bath, the potency of the mixture will remain constant and in theory an be used indefinitely.
He's applying the electrolyte with a soaked electrode vs. a bath and from experience that is going to destroy your lead clips if you plan on doing this for more than a couple of etches. Me and Haley did some work in the past with copper sulfate baths for etching copper (that's root killer from home depot, super available) we used soaked electrode methods back then, but the bath mixture allowed a consistent etch on the entire surface at the same time. Much less labor intensive and time consuming.
Speaking of labor intensive, I have vinyl and you can hand cut it if you really really want to, but I also have my own vinyl cutter at home so we can use it to rapidly produce detailed resists. No art experience required. Of course there is also a cutter at the space, if you have the time and inclination you can use it just as readily.
There's another method I have been itching to try and if we have enough interest we can figure out a time to explore it together. It is a passive etching technique for large surface areas that requires no external power source! You basically create a wet cell out of felt soaked in copper sulfate. Just spread it out over the material and Wham-O!