Why map png files are required to generate map record

Greetings,

I just begin to work with nuscene dataset. When I use this function:
NuScenes(version=version, dataroot=dataroot, verbose=False)
I found the images of maps in .png format necessary for generating map record. I did not understand why the map images are required to generate map record, could you please help? Thanks!

                                                                                                                      kind regards