It was the (international!) law, but it would have been a good idea even if it hadn't been required: beginning in 1929, radio amateurs had to build their transmitter to meet nominal requirements for stability and spectral purity.
The rules weren't especially stringent; existing designs could be adapted and a beginner's transmitter wouldn't cost any more, though it might take more time to adjust. The benefit was that sharp, stable signals get through where wobbly, "squeggy" ones would prove too hard to read.
...It happens that for radiotelegraphy, a "1929 type" transmitter still makes signals stable enough and clean enough for amateur use. Even if the other ham is running a software-based radio on a plug-in computer card, he (or she) can tune it in.
And you can build one! Canadian ham VE7SL shows how, complete with original schematic, recent photos, modern schematic and ways to couple it into your coax-fed antenna, and then ends with a photographic round-up of 1929-style breadboard rigs by a dozen-odd builders. (And if you'd rather build Ross Hull's wonderful 1929 Hartley, why, he'll show you that one, too!)
I'm going to have dust mine off. H'mm, I need to find me a type '45 or '10 transmitting tube.*
* The ' in place of a first digit replaces the manufacture's ID number -- "2" for RCA, for example.