It wasn't actually that common, but Sears-Roebuck (remember them?) did that with a lot of their Silvertone products. They even included parts lists that people could order from.
What's important was that publishers like Sams (via their Photofact service) provided detailed information to repair people and technically-oriented owners at reasonable prices. They still do, but their stuff is quite pricey now and was occasionally inaccurate.
It was pretty common. RCA, GE, Dumont, and many others had that practice. It started to fade once transistor based devices starting pushing tube based stuff aside. The SAMs docs picked up the slack.
It served us well for something like 10 years. I bought a new one $400 and got that installed and working before the second hot and muggy night on Thursday. Then I started digging into the old one. Found a bad thermistor, didn't respond at all to ice water or a hot lamp, the other one responded instantly doubling the resistance.
Ordered 2 from Ebay $12.50 free shipping. If I get it working I'm going to have an air conditioned garage.