Suppose I use program foo as part of some enterprise application on a server. I am inadvertently relying on undocumented or undefined behavior of foo.
An update to foo changes what happens in that undocumented or undefined case, and with the new behavior my application does not start correctly.
There is nothing the package manager can do to ensure that this does not happen because there is nothing wrong with anything the package manager is managing. The bug is entirely in my code. All the update did was expose it.
The question them is when will that now exposed bug actually get hit, so that I become aware of it and fix it.
The purpose of the reboot is to make sure that exposure happens at a time when it will not cause much harm and I will not have a lot of trouble finding it.
An update to foo changes what happens in that undocumented or undefined case, and with the new behavior my application does not start correctly.
There is nothing the package manager can do to ensure that this does not happen because there is nothing wrong with anything the package manager is managing. The bug is entirely in my code. All the update did was expose it.
The question them is when will that now exposed bug actually get hit, so that I become aware of it and fix it.
The purpose of the reboot is to make sure that exposure happens at a time when it will not cause much harm and I will not have a lot of trouble finding it.