Q:

Why do things tend to break when one doesn't restart after kernel update?

I know that it's strongly recommended to restart after updating kernel (well, anything important) so that the changes can apply and can propagate. I also know about the option to livepatch kernel.
However, I'm confused about why do things tend to break when I update kernel and then don't restart. It's pretty much random, sometimes it happens and sometimes not, and reboot always fixes it. Sometimes it's bluetooth refusing to connect to new devices, netflix cannot play videos and tells me to restart computer - things generally get weird.
But why so? I don't want an answer why my bluetooth stopped working, however I'd like to hear a high level explanation why do things in general break?
Is it because part of the update is applied and part is not, maybe there are even multiple versions of something running at the same time and things break when it tries to communicate?
Why do things get weird after kernal update?

A:

Some, drivers and services are dependent on your kernel's version (virtualbox's driver is one of them). When you update your kernel, some services are stopped to be updated but cannot be restarted because they rely on the new kernel. Most distros don't patch the running kernel (Live patching) to upgrade it and instead prefer to keep the old kernel running and boot the new kernel on restart. If your bluetooth driver required the new kernel after it was updated, it will remain stopped (or in a crash loop) until the new kernel is loaded.

