I guess I'm going to be a bit of a contrary voice in this thread.
For context, I've been building PCs myself and for others (and for work at times...) since the mid 90s (486sx or dx? check out that ISA bus. Don't forget to set the jumpers right!). As most here have pointed out, custom building is often cheaper, offers greater customizability, greater upgradability, and better repairability, than pre-build machines. You also can have more pro features (raided disks, etc etc.) These are huge wins for many people, and it's why I still build my own powerful desktops.
However, I am increasingly recommending pre-builts with warranties from respectable companies like Dell and Lenovo for many people. There are several reasons for this.
- computers are fricken complex and parts / drivers all have compatibility concerns. Has your exact combination been built and tested by someone? If not, you may have stability issues that are hard to resolve, with everyone pointing a finger at someone else. Probably your PSU. No wait, your fault for picking X brand motherboard. No wait, must be that graphics card model. Once you hit this point, you're SOL. A pre-built has a well tested configuration.
- if your custom-built breaks, or has issues from day one, same as above. Finger pointing. I have a PhD in Computer Science and still can't convince support people that I know their ram is faulty. If your Dell has those issues, you're not worth the hassle, they'll just replace it. And if you have an upgraded warranty they'll send someone to your house within a couple of days to fix it or order new parts.
- how busy are you? how much does futzing with a flakey PC on your Saturday ruin your rare time at your hobby? For me, I like it and have fun playing with the bits. But for many the risk of losing precious time is a huge problem.
So, for me, I still do custom builds. If you're not into that, and you have a good shop that does them, that can be great as you'll have service. But it's not a slam dunk, and there are plenty of good reasons to go with a pre-built machine.
PS - I checked out that youtube video and unfortunately it's pretty sensationalist with some.. let's say colorful exaggerations? can't blame him for wanting clicks though