Back in the day, Willy ........... wayyy before my time, there was only "trade" paint. DIY didn't exist. Decorators used to thin it, colour it, etc, and generally mess with it to get it the way they wanted it.
It's kind of a leftover from that. Apart from trade paint maybe having a slightly greater opacity, and more pigment, between you and me, I think it's just marketing today. I guess it's to make the punters think they're getting something special.
Stick to the good brands and you won't go wrong. Though, even the "DIY shed" own brands are now generally made by Crown or Leyland, or something similar.