These wires were needed in all Trinitron products, but the more exacting demands of high resolution (1024 lines!) meant the support wires were quite visible horizontally, especially on uniform backgrounds like a gray Windows desktop. They divided the screen into three equal areas.
Despite this apparent flaw, Trinitron still had huge market share amongst enthusiast PC owners. Maybe it was brand loyalty, or maybe they really were much better quality than any other CRT out there?
At least as I remember it, if you wanted to take (or be seen to be taking) color reproduction seriously, then you had to have a 21” Trinitron. I guess the black horizontal support wires became just as much a part of brand signaling as the RGB lozenges in the logo. Lots of people I knew had amazing Trinitron CRTs and all they did was edit Word documents!
Kind of like a Leica red dot, or the white spot on Dunhill pipes.