It always surprises me when people say that :)
The thing is, background MIDI on web pages was sort of an accidental quirk in the history of the protocol. Its primary function has always been its face value; it's literally in the name: Musical Instrument Digital Interface. And that's where MIDI has dominated since its introduction in the 80's.
Which, in some ways, is not for the best, because a better name for the protocol would have been Keyboard-Driven Instrument Interface. MIDI messages (to the extent that they are standardized) aren't good for anything else.
ROLI Seaboard is an example of pushing the protocol to its limits. The original spec didn't have polyphonic pitch bend (so a continuous slide from a minor chord into a major one is not possible); ROLI gets around it by transmitting each note one a different MIDI channel as if it were a separate instrument.
This example is just one limitation, but there are plenty; there's no good (read: standard) way to control a stringed instrument (or its emulation) with MIDI. You can play one note on a guitar in 4 ways (on 4 different strings), but MIDI isn't specced for that. Horns are passable because they are monophonic, but controlling performance aspects is up to the software. Drums get away by assigning different MIDI notes to different ways of playing a drum. Etc.
The thing is, the protocol was made just barely flexible enough that it lasted to these days; and the ability to drive an 80's synth with a controller built today (and connect them all to a modern computer/iPad/etc) is an amazing feature.
Because of that, MIDI underpins all modern digital instruments and music software. And because of that, anyone who makes electronic music is a keyboardist (or at least has a MIDI keyboard). The word MIDI controller pretty much defaults to MIDI keyboard.
Want to really have your mind blown about where MIDI shows up? The shape of the protocol defines the shape of the music. Just like different programming languages encourage different styles, so do musical instruments and their physical interfaces.
And so the absolute majority of electronic music today is written from a keyboardist perspective.
You won't get to hear a minor chord continuously slide into a major one in pretty much any music you hear that was made with digital/software instruments/computers.
Because the piano can't to that.
And because of that, the keyboards can't.
And so the synthesizers don't.
And so MIDI doesn't.
And so nearly all music software, for decades, wouldn't. (Some does, but it's not standard, so no mass adoption.)
And so your music doesn't have it a major chord slide into a minor. Because even if you don't play keyboards, your interface to digital instruments is still a piano roll.
Because MIDI.
I'm really looking forward to MIDI 2.0.