chown changes the file owner. chmod changes permissions. So, if a file or directory is owned by root but a user should have access, you could make them the owner or you could keep root the owner and just allow read/write access.
They come up more on servers where you often have multiple users with different access levels. Some users might not have sudo permission but do have full control over their home directory and whatever else they need. And web servers, for instance, will usually have a user called www-data or similar and it’s shared by all the users in the “developer” group.