The reason for ZMH was that a node making a connection to another node required the use of a physical phone line where one node would dial the other, which would stop anyone else from connecting to those two nodes. So if you had a lowly user logged on to your BBS, noone else could log on to it, and you couldn't send or receive Fidonet mail.
Contrast that with TCP/IP where it's possible to have millions of active connections to other nodes on a single node without breaking a sweat, and it's just a ridiculous comparison.
Another hilarious quirk of the times was that almost noone was running a multitasking OS (Amiga users: shut up!), which meant that once someone had called your Frontdoor system and delivered mail, it would exit, and start your mailer (Fmail!), which would uncompress the received file, put all the messages in your local message db (We're talking flatfiles of course, SQL was nowhere in sight), and prepare packages for all your downstream nodes, and after that it would start up Frontdoor again, at which point you could finally receive calls again.
So not only were the systems unable to handle more than one connection at a time, there was also no background processing, so while your computer was busy unpacking an enormous serveral-hundred-kilobytes zip file of mail, it couldn't accept any calls.
Oh, oh! And there was no Unicode, not UTF8, so there was a years-long technical debate/flamewar/ridiculous shoutfest about which character set to standardize on, where the main contenders were CP437 vs. ISO-8859-1.
Looking back, it's amazing it worked at all, but it did, and that was fucking magic.
