Can you answer why glass primer is black and why it is essential for urethane adhesion to glass?
Not: primer makes the urethane stick better to the glass. another reason.

From my understanding it etches the glass giving the adhesive sometihing to bite into

Yes but there is another important reason

Urethane degrades in UV light.

DING..DING..Ding yes the uv rays gets past the frit and erodes urethane adehion to the glass.
5 attaboys goes to owner.

but what about urethanes that don't use primers, rather, an activator that is clear? Do they still have UV protection?

Right, I was going to comment most glass primers or activators are clear, good thing for the frit band. And even primerless to glass urethanes like essex, 418hv, I believe still require an activator or primer. I would never go primerless!

One of the reasons "primerless to glass" urethanes are slower to cure is because they need that extra time to etch into the glass. Quality products like Dow's U-418/418HV work just as well as any other high-end product they offer. Hell, I'll go as far to say as there are many brands of urethane on the market and when it comes down to it, they all work. The key is using them properly. Installers should be certified by the urethane manufacturer that they use and updated at least every year if not semi-annually to ensure that they are up to date on new products and procedures.

the glass frit was brought about because of urthene breakdown. you never been dirty till you got into urthene that had broken down, and powdered out. ulgy, ugly, ugly. you need to prime(black) unfrited glass if you plan to use urthene.

glass with frits also let uv light pass too. (not all but alot of them)