It used to be a de facto standard in many programs. Since almost no mouse had a scroll wheel, you'd use the space bar or the cursor keys. Spacebar was usually faster, I guess some people still do.

I do this too. The pattern probably dates back to first Unix pagers, or perhaps to the paper era.

Still doing that, also in Thunderbird, to scroll through E-Mails and go to the next one when reaching the end (or pressing "n" or "p" for previous). I even use shift + space to go up again. I thought it was very common. Another alternative, maybe a bit more intuitive is using page up and down buttons.