Look at GUI/Hide section.
Okay the section you mentioned shows how you can configure Clover to hide partitions. Technically you answered the "why" by saying the "partitions are there because you didn't hide them."
However, it doesn't answer the original question of what the purpose of the partitions are. But I don't want to take up more of your time because I know you are busy supporting a lot of guides, which is more important to this community than teaching someone how macOS works.
So for anyone else curious, the answer from a combination of ******, insanelymac, stackexhange says: VM is a virtual memory space so that apfs can perform swap, similar in function to the swap file found in linux. Preboot is a partition that manages boot and you can't boot without it. Both are new features of apfs that are not present in HFS. Anyone else free to enhance this answer.