With the rise of cheap 4k cameras, it goes faster than you think. I download guitar training videos where a 25 minute tutorial can be 2-3gbs. I believe those are “only” 2k resolution. If you do not download them in a space conscious format or recompress (using mildly aggressive settings can drop them to 100mb), a handful of videos can quickly fill a hard drive.

Why wouldn't one download something like that in 720P video/196kbps audio?

I mean, there's a time and a place for 4K, but watching the zits on the face of a guy who tells you how to play C Am F G on a guitar isn't it.

Not to mention, most of those cheap 4K cameras won't have optics to utilize those pixels; no quality will be lost in 720p.

My downloader defaults to the highest quality, so unless I remember to specify a setting, I get gigabytes of video for something that could easily be 480 quality. In the event I forget, I reencode with ffmpeg.

It's a reasonable default, but if one has gone to the trouble of setting up a NAS and scripts, might as well tick that box.

That depends, if the source is bright enough, those little pixels will have enough signal to work effectively.