Of course, like every one else, I’ve used the Line Tool to draw automation and CCs. But I never knew until today that you can use it in the piano roll editor (and, presumably, in the other editors - I didn’t test it).
Of course, again, all you pros out there knew this, but I didn’t.
Trouble is, I can’t think of a use for it.
Wait - I can use it to draw… Oh, anyway. New trick, there ya go.
Well, I just learned about this tool … now. Glad to join the “everyone else” crowd. 
From your video example, it looks like it could be useful for drawing a glissando. Of course, I’d generally just play those if doing them on a piano part, but maybe if editing a part if some sort of quantization screwed one up, it might be a way to fix things. Or if just wanting to create something of the sort over a specific range of times… And I think you can “quantize” notes to a scale or key, so it might be a way to get something like that without the in-between notes you might be likely to get by just playing a part?
I thought “arpeggio” but that usually means “notes in the scale”. But “glissando”, yes, I sometimes use those in organ parts.
The thing with arpeggios, at least as I typically use them, is that they usually aren’t one-time thing like an organ or piano glissando, but tend to go up and down, and sometimes not in a straight line but more like finger-picked guitar patterns (probably what I do most frequently on that front, whether on piano or with an actual guitar sound).
In general, I almost always play parts rather than writing them in, just using the key editor to adjust from there, if needed. So my likelihood of using the line tool for notes is probably minimal. However, it’s good to know about it for CCs. I’ve just been drawing those in, but between my limited mouse drawing skills and Cubase’s reducing the number of nodes in ways that often don’t get the same sort of shape that was emerging while I was drawing, maybe this could make for faster tweaking when dealing with adding expression and modulation. Maybe… (I’ll actually be doing that for a virtual lap steel part, for the volume pedal, later today, so it will be a good excuse to try it out if just overdubbing riding a fader doesn’t get me what I want.)
I think I’ve only used that feature once and it was a long time ago. So many things have been forgotten. Thanks for the reminder.
The Line Tool allows for a very fast way of getting a 16ths hi-hat line for example:
Of course, after creating the notes you can use the LE to vary velocities and the quantize settings for swing and other shenanigans.
I would use the drum editor for this, however… For those chugging heavy metal rhythm guitars that I often use, it may be, er, useful
The post I literally read just before this one showed a scale based mode to show Notes in the Key Editor which would be pretty useful used in conjunction with the Line Tool. While I’ve known for ages you could use it for Notes, like @Googly_Croft I never saw a real use case.
Cubase really needs scale degree transposition - #6 by Johnny_Moneto
Another use case:
Listen to the orchestral built-up between the main sections of “A Day in the Life” by the Beatles.
The Beatles had to jump through hoops to make classically trained musicians improvise for a couple of beats with just two directions being given to them: Go from the lowest to the highest note AND do it in your own time ignoring your neighbours to the left and right.
With the line tool you can just draw away like you’ve never been drawing before (no need for red noses, tuxedos, ballons, etc to cheer up the orchester).
Of course, I’d always prefer the real thing over samples, you just can’t beat a live performance.
Awww.
I was thinking of that example yesterday but couldn’t for the life of me remember the name of the song at the time.
Side note: Back in 2010, a guy who was local at the time hired me to play some keyboards on a trippy instrumental he called “Classic Mushrooms”. He was (still is) a huge UK music fan, including the Beatles, and, in addition to just some general organ and string patch stuff, he wanted me to do a few specific bits that referenced Beatles recordings, one of those being that specific bit from “A Day in the Life” (the other was a specific electric piano part from “I Am the Walrus”). I remember researching what you mentioned regarding how they did it as part of my preparation. The execution was decidedly different (and not as effective).