I read about registered memory and noticed that RDIMMs(and also LRDIMMs) are arced:
What is the purpose of that? Why are RDIMM/LRDIMM modules arced?
You're referring to DDR4 DIMM modules, who feature a curved shape as noted in your photo.
This is new to this type of RAM. Previous module standards did not have the same curved edge.
The modules have the curvature in the edge connector so that the pins are not engaged at the same time during module insertion. The reason for that is to lower the insertion force needed to seat the module in the (densely situated) motherboard slot.
It's there to help during installation.