Most often it's a case of minor patches to the audio system, or recognizing a card reader (both of which are weirdly often unique per model, often even on sub-models), and these are patches that do make it into the older kernel. That process just takes time, and they want a usable image right away, so they get something specially patched that will shortly be replaced by a mainstream kernel update. For extreme cases there's dkms, where you can have a package that replaces a kernel module from source that is rebuilt on kernel upgrades.

Canonical used to manage PPAs and packages to handle this - keeping them updated and then eliminating them as the fixes appear upstream. The PPA packages also contain trivial things like tweaks to get the keyboard hotkeys to work right.

I know this lives on because I have a Thinkpad and the vanilla Ubuntu installer adds "sutton" packages to tweak its install, and sutton is the internal name for Lenovo enablement (although the package has nothing active in it, so presumably vanilla just works by now, which I'd hope, as it's a 3 year old machine).