Why are there some extra letters on lego Car License Plates? Most of the car in Lego City have a sticker license plate. The number on a license plate is corresponding to the Lego Set number. But what about these extra letter there? Is it just to add more realism? Are the initials  of the particular designer?

See e.g. here:
Police Car - set 60068
Crooks car - set 60070
The letters typically represent the initials of the set designer. See more discussion on this topic in this EuroBricks forum thread.

For example, the initials "MB" stand for Marcos Bessa, where "MS" is for Mark Stafford.

Some other sets also carry set designer initials, sometimes subtly incorporated in the build (I believe Jamie Berard, the famous Modular Houses designer, puts his initials in some of the builds), other times on the lettering on the sticker sheet. It is definitely not standard practice.