XMP sidecar - I don't get it

May be there is a basic explanation for the XMP sidecar options - but there a lot of threats ...

When using the the Auto Sync options: load and full sync?
I guess, the load option means it reads if there is a sidecar file - full sync means it also writes into the sidecar file.
Does C1 this every time or just when importing an image?
Whats the benefit of a sidecar?

What is the XMP and IPTC option for?
Why is there a second option for Sidecar XMP instead of metadata?

I think C1 stores the all the metadata inside the catalog/dng file anyway and the sidecar is just creating troubles?

Thanks for explaining!