Why vi? I've never understood the draw to that when there were more intuitive text based editors available these days.
My take on why vi:
No matter what UNIX-like operating system you find yourself using, chances are you'll find vi or a vi clone (like vim). I, personally, have run vi or vim on all sorts of computers and operating systems:
1) BSD 4.2 on a VAX 11/750 (it's where I learned vi and UNIX back in the 80s)
2) VAX/VMS on VAX 6000 series, 3000 series, MicroVAXen, etc.
3) OpenVMS on Alpha/AXP systems
4) Digital UNIX on Alpha/AXP
5) AIX on IBM RS/6000s
6) HP/UX on various HP machines
7) Solaris on TONS of different Sun Microsystems machines, from desktops to big Enterprise 35000 servers.
8) SUSE Linux on IBM mainframes
9) Windows machines from 3.0 to Win10, including everything in between.
Haven't been able to do that with nano. EMACS comes close, but EMACS is freakin' HUGE and is a performance killer. (We had to tell the developers they couldn't run EMACS on production servers, as it tanked performance. We didn't just tell them, we proved it to them with hard data. Had to go to the CIO to get them to stop. They started using vi shortly thereafter. Nary a problem with performance.)
There are some files in /etc that expect the EOF to look a particular way. (Forget the details ATM.) Some editors tend to mess that up. Vi always ends the file the right way and you have no problems. Took over a week to figure that out at one point.
For me, it was the first full-screen editor I learned. I tend to "think" in vi. I'll install VIM on any non-Linux/UNIX machine as one of my first tasks, even work laptops.