Real first name! I was a Starbucks barista back in college. This is very useful for keeping orders straight when it is busy. If 20 people are in line and the baristas are running around trying to make the drinks, and 7 of those people ordered a grande latte, that are all slightly different... you want people to get the specific drink they paid for. After all, they are paying to make sure it is 2% no foam sugar free vanilla etc. Sure, this is marked on the cup but 1) it is often blocked by the cup holder and 2) people are much more likely to pick up the right thing with their name on it than the barista drink making codes.
That kind of situation happens all the time. Sure you could give order numbers, but names are a bit nicer.
Also, if you go in the same time everyday, you will probably see the same baristas. Then, they can write your name without even asking and it is cozy.