Bee, my experience is limited, so I'll answer your question, as best I can, but it will only be my own mindset. Sure as heck cannot speak for everyone. There are far too many goals in breeding. For some, it is the show. For others, it is the preservation of the breed, and for others, such as myself, it is a a sincere desire to be faithful to those who bred the breeds. For utility breeding, little of this applies, but when keeping and breeding heritage birds, I consume as much information about the life, times and writings of the breeders who made these breeds 100 years ago or more. These pioneers actually did some fascinating writing. The SOP reflects much of that history, in my mind.
With that long and boring preamble, these birds are precious not only because of their "looks", but precisely because they also incarnate the personality of the original breeders intent. Forage? Well, duh!! What kind of heritage dual purpose fowl, such as the Barred Rock, doesn't forage? Gentle, self confident, regal? Yup, all that stuff in spades?
How about broody? How often does a hatchery or utility Barred Rock go broody? Rare as hen's teeth. These ringlet BRs are broody as all get out.
That Red cockerel I pictured above? He's a profoundly capable flock tender and he's just a pup. I've not seen those skills in a utility/production red. How many posts are here on BYC that say "My RIR roo is aggressive and attacking me!" blah blah blah. Then, you realize that they have a hatchery production red, of course. All over BYC there is the prevailing opinion that RIR roos (I use that term intentionally) are mean and aggressive. Truth is, they aren't Rhode Island Reds at all. They are mean red mutts.
NYREDS was asked about his heritage, heirloom Reds. "How do you get calm and friendly Red cockerels?" Know what his answer was?
"Just hatch them". LOL