It seems like the purpose is to have the law and all the paperwork set up as a precaution for the future. Sure, right now it’s all voluntary and just rubber stamping, but if in the future they need to do something like Ukraine and lock down travel for military aged men, it’s much easier to flip a switch and start denying travel permits rather than having to set up and fund an entirely new system for requiring travel permits.
Also, in that case, they know where all the men currently are, which is probably the main reason to implement it now. m
If it were to get closer to war (i.e., Spannungsfall, let alone the Verteidigungsfall) a set of laws would unlock that allow control of various areas of life and the economy anyway.