There are all sorts of legit uses. There are some videos I've uploaded that I've long ago lost the original source to. Also, if I wanted to go home and see my parents, it's probably better if I download the ted talks, youtube videos etc that I might like to watch on my personal connection than theirs.

Valid question! On first thought, you would think the only reason to use it would be for media you don’t own (if you own it, you’d already have a copy). And you are probably correct. However there are valid reasons for doing so: many people have posted here that they use it for journalism (fair use) among other things.

I do remember years back, LinusTechTips admitted that their video archives were just on hard drives scattered around the building. So if they needed a clip from a previous video, they’d just download the video from YouTube and “crop” down to the clip they needed.[a]

[a]: They mentioned this in a video where there were building their “Vault” server that eventually turned into “Petabyte Project.”

I use it to watch YouTube videos in MPV. I have a machine where Chrome won't start and Firefox can't reliably display video without dropped frames, but MPV runs perfectly.

There are many creative commons videos on Youtube, and certainly downloading those is within the intent of that license?

In the case that you were more concerned about the legality than the morality of it, you have to remember that downloading freely available videos on YouTube is perfectly legal. (Of course sharing the downloaded files is not, unless you have a copyright on them, or if they are in the public domain)