Looking at amateur radio forums and email threads, oftentimes amateur radio operators will use "73" or "73s" where one might otherwise expect to find "Sincerely" or "Regards". Why?
The usage of "73" for such a purpose comes from the Phillips Code, originally devised in the era of telegraphs to speed up transmission of common messages by mapping them to numbers.
"73" mapped to "best regards" or "my compliments" and was intended as a general valediction for transmitted messages.
It is still used for that purpose today in morse code transmissions and often makes its way into everyday correspondence among hams as a form of jargon.
There is also "88" (which maps to "love and kisses") that sometimes takes the place of "73" for communications between affectionate parties.
Most of these things come from morse code. They were used because they have a distinctive and musical signature, listen to the rhythm of '73' some time. Rhythm was (and should still be) the mark of a good operator. Alas, some non military operators in the early days were not, that is where 'ham' came from: because the operators sounded 'ham-fisted'. When I was a young boy my navy father taught me an exercise for learning rhythm it is: best bent wire / g Try it out some time on your morse key, you will be surprised how it improves your musicality.
See a list of jargon here: https://www.qrz.com/page/hamspeak.html
73 de TF8HA