Why? Back in the day you could output a text file directly onto the printer and it would print, possibly interpreting control characters like backspace or carriage return to overwrite existing characters.
For that, the printer would also need to interpret the characters in the correct character set.
But who would design a character set that only works properly on printers in 1985? Hence the question of whether and which operating systems properly supported that kind of use.
It's still a pretty crazy idea to add something that only works on printers at that time. It's very much worth asking about OS support, and "it's there because printers" leaves a lot of story untold, it's not a sufficient answer.
We did printing with heads on specialized form paper. If you wanted to change the font you opened up the printer, took out the head, and replaced it with a new head. The character set standard defined how the head was physically manufactured. You want all the printers in your fleet to produce identical output.
In 1985 "OS Support" meant picking up the phone and talking to the vendor directly.
For that, the printer would also need to interpret the characters in the correct character set.