In my experience the majority of first time and mid-level managers are hired from within the company. There is nothing wrong with this. However, when this happens, the hiring manager or new supervisor makes a lot of assumptions and cuts a lot of corners in preparing the new hire to be successful.
The experience and what the individual contributor knows about the company is much different than what you, their current manager, knows about the company. Why shouldn’t an internal hire get the same orientation as an external hire? Now–5 to 10 years later and with a new role–the internal hire will be listening for and hearing very different information than they when they first attended.
Just because the new hire “worked for you” for 10 years does mean they actually know what you do. Too often I see people start jobs without job descriptions. Obviously, a new hire wants to know what they are supposed to do. They might ask–maybe not because perhaps it is supposed to be obvious–and when they do they get a spur-of-the-moment-whatever-is most-important-to-you-at-the-moment description. And then at the end of one year–someone is going to be responsible for measuring their success. Will you remember what you said was most important for them to achieve? Did they write it down? Was that spur of the moment burst of insight really what was most important?
Likely during the interim, the new hire will be lost without a job description. They want to be successful. They have been successful before and likely they’ll go back to doing that again. But that is not what they were hired for–what were they hired for again?