Computer nerds are self sufficient when it comes to fixing their computer. Non-computer experts have to find some other person with greater computer knowledge to repair their computer or phone. That person will then be able to access all data stored on their computer or phone.
By handing over their computer to a third party, such as a computer repair shop, they are giving their personal data to a stranger. And it is so easy for that stranger to access this data. So they will.
This is not only true for computers, but especially for phones. If you are a women, you should be extra concerned. It is so easy to obtain access to your photos. And people do.
The only safe thing to do is either:
Both actions will make it impossible for the computer technician to resolve any operating system or software related issues. Also, it will be harder to diagnose hardware failure. And if you erase the computer, who is going to reinstall it?
A third option would be to implement a secure file container where a user would put personal information. But this concept is way too hard to understand and implement for most users.
So in the end most people must find a person they can trust and who is willing to fix their computer. But that is never a safe bet.
So assuming that you must trust your computer to a person you don't know too well, it is smart to never store any content, especially personal pictures or videos on your computer that you would not want them to see.
I had to turn in my iMac for repair because the internal hard drive was dying. So I erased the entire disk by overwriting it with zeros. This takes a few hours, but it guarantees that my data will not fall in the wrong hands. Honestly, I don't have any data I'd really want to hide, but still, it's my data and I don't want it in the hands of unknown people.