Probably because there is a method within Discourse (the forum software) for bringing a post to the attention of an individual user - add an “@” to their user name - so @Bosendorfer for instance and they will be sent the post as an email.
It’s a bit more focussed than just replying to someone’s post - and it can be someone other than the author of a post to which you are responding.
But, obviously, it helps to know the user name as well as the full name to make use of that feature.
I didn’t see that question in the OP but @anon88169868 answers it below. Basically, it’s easier to tag people with a short name. The software also allows removal of the real name field in which case you would see only the short name. But we don’t remove it on SF.
Mines “Regular”
It’s the healthy diet that does it
As a former nurse, I learned that ‘regular’ means anything from twice a day to once a fortnight. 
Mine too - hatched, matched and dispatched.
Twice a day I can understand, but once a fortnight! Must be “grunt ‘n’ dump… praying to all Gods imaginable”! 
Yooman diversity is as diverse when it comes to crapping as yoomanity itself. Believe me, some individuals serenely go as long as two weeks without a dump, and it is indeed then something of a marvel to behold (at a safe distance). And remarkable in its silent efficiency… 
I’ll send you a box of my home grown beetroot I bet you wouldn’t last 2 days let alone a fortnight.
How did this thread descent into toilet humour… 
well, as were here…
Lateral thinking
One of my bosses once nearly went to A&E with a severe recital bleed until he remembered he had had nearly a jar of pickled beetroot with his hot pot
Very kind Wozza, and, we grow our own. Beetroot always strikes me, as the most amenable of root vegetables, such a placid and undemanding plant, co-operative in every way, easy to cook and peel, and delicious a la vinaigrette. I can easily eat a dish without accompaniment. And pickled it keeps for ever. Betterave brave!
Easy lunch. Corned beef, chips, beetroot and home made chutney!