We used it to discover how easy it is to replace elements in an electric oven. Otherwise, DH would’ve called repair guy. Instead he did it for what the parts cost which was <$25.
DH also used it to figure out how to get the hard drive out of a tower that he just couldn’t get opened.
As an above poster stated- numerous things. Still have a couple of old fix it books on the shelf but it is so much easier to go online and look at several videos. Sometimes it takes a bit of searching and combining of videos to get the info desired, but so much easier than in the old days.
I used it to learn how to mitre the corners of quilt binding and join the ends. I’m not good at spacial relations so I was able to rewind and replay numerous times until I got it. No human friend or teacher would want to stand there and show me over and over. And over.
YouTube showed me how to cut/chop/seed seriously hot peppers. Also, learned how to make hot pepper jelly,
The ballet bun post reminded me. I don’t have all the traditional “girl” skills. When I was planning to be on a 10 day hike with no shower available, I realized it would be easier to french braid my long hair and not have to touch it during the hike. But I was hair challenged and had no clue how to french braid my own hair. YouTube to the rescue!
I bought a bicycle lock that uses a four-letter combination that you choose yourself. I chose a combination and was positive that I remembered it correctly, but I couldn’t get the lock to open–very frustrating, considering that I had only just bought the thing. Then I turned to YouTube, where I got a lesson from a 13 (?)-year-old on how to pick a bike lock. It worked perfectly.
There are many different types of ballet buns, one of which is called a “cinnamon bun.” 
I used YouTube video to fix my ice maker. All it took was a blow dryer.
I also wanted to learn how to fold my towels the way hotels do. YouTube was great for that.
I used it to replace my headlights and taillights on my minivan, and to replace the toilet tank innards that the plumber was going to charge $200 for (my cost maybe $10-20). Soon I’m going to use it to replace my bike inner tubes.
You can tell, no ballet in this household, @tutumom2001!
Learned how to replace the headlight on my car.
I have been “homemade pie crust” challenged since the beginning of time. I’ve spent hours online reading tips and trying different recipes that others have had success with, but still seemed to have difficulty because the YouTube videos would skip the critical step for me, which is how to get the pea sized pieces when mixing the shortening with the flour. FINALLY I found a YouTube video that filmed the process from start to finish, plus it was filmed from above, making it very easy to see what needed to be done. I, yes me, Flymetothemoon, made a delicious, flaky crust. And it was easy, just like everyone else said it was, but could never explain the process.
(By the way, the secret is in the way your fingers manipulate the shortening and flour, which of course all of you bakers already know.)
Teenaged sons figured out how to tie ties and bow tie under deadline. Spouse was out of town at various key "requires a tie or bow tie (orchestra) moments in their lives, and I was no help. They pulled up youtube videos, stood in front of the mirror, and did it. Alleluia.
How to set a mousetrap.
Just this week my son and I repaired the dryer! A couple of weeks ago we learned how to change the sand in the pool filter. We repaired the refrigerator awhile back.
“(By the way, the secret is in the way your fingers manipulate the shortening and flour, which of course all of you bakers already know.)”
Another secret is lard. Grandma knew what she was doing. 
I looked at a bunch of flashing with stucco details to make sure that what I was drawing made sense. I’ve used it for dishwasher stuff too. Younger son watched a lot of origami videos back in the day.
My all time favorite is how to fold a t-shirt in two seconds: https://www.youtube.com/watch?v=An0mFZ3enhM
How to properly install a bat house. We now have itty bitty babies hanging in our bat house! Did you know that a house for little brown bats needs to face south and be close to a source of (still) water? We had no idea!
Refilling the liquid fertilizer tank on our irrigation system, programming the pool equipment, fixing appliances, plumbing tasks, cleaning tips,computer mysteries, knitting techniques - too many things to count!
Almost every “how to” computer issue that comes up at work.