This is standard for all Linux computers (and probably others). (So this is not a Pi specific question). In the bad old days you had to manually generate keys.
For most users these are only relevant for
ssh although they are used in many places to uniquely identify the host.
The one aspect of the Pi which differs from "normal" computers is the ease of moving SD Cards between machines. There is no problem in having identical keys.
Indeed having different keys can cause problems, as using different keys will cause problems if you have multiple SD Cards as the MAC & key combination will cause other machines to identify a problem.
I go to some effort to make sure ALL my SD Cards for different OS have IDENTICAL keys so I can effortlessly swap SD Card/OS and only change every 2 years with a new OS.
In short it is unlikely to make any difference for most users.
It is a simple procedure to change keys if you want them to be unique. I use the following to Recreate ssh host keys:-
sudo rm /etc/ssh/ssh_host_*
sudo dpkg-reconfigure openssh-server
If you want to explore further https://unix.stackexchange.com/ is a better place to ask (although many would consider duplicated keys a security violation).