I have read that I can also apply to the Double-Degree Program before getting enrolled in the Physics Master at Freie Universität Berlin. What is this good for?
The purpose of the early application to the Double-Degree Program shall help those candidates that are particularly interested in such a Double Degree to decide at which university they should enroll.
If someone applies before April 30th to the Double-Degree Program along with the regular application to the Master program in Physics at Freie Universität Berlin (deadline for the latter: May 31st), and is accepted to the Master program, he/she will know about the outcome of the application to the Double-Degree Program before he/she has to enroll to the regular Master program, and thus can decide at which university he/she would like to enroll.
Successful candidates of the early application will get a provisional admission to the Double-Degree Program, which means they are admitted if their average grade in the first semester of Master studies is 2.0 or better. (If the grade is worse, they have to apply again after the first semester of the Master studies.)