Back in the day, the phono input on stereo receivers was qualitatively different than the other inputs were. That said, turntable manufacturers are probably well aware that today's consumers might not have access to true phono-ready equipment.
Back then, mismatching the source and the input sounded terrible. The other sources might be a cassette deck, a reel-to-reel deck, a dedicated AM/FM tuner, and perhaps even a microphone or guitar. There were, however, standalone phono preamps that would turn the signal into something the other inputs would have no problem with.