The same reason we care what your house wears as a habit - so we know who is what.
The FSSP is in good standing, and their mode of dress helps those identify their station. Well, if it were distinctive, it would. Since it isn’t, it merely is indicative of their clerical status (since non-liturgical wear of the cassock tends to be indicative of either clerical, major or minor, status).
The Roman collar is even less indicative, as it’s worn by so many religious as well, and by many protestants.
A uniform has several purposes:
- Identification of members to members
- Identification of members to non-members
- Indication of rank or status
- protection from the elements
- modesty
- triggering of particular mental states in the wearer
- triggering of particular mental states in the viewer
The habit is a uniform. The clerical dress is a uniform. Even vestments are a uniform.
Your habit marks you as a man apart - a member of the Franciscan community, and possibly of a particular house. It identifies you to the public as a consecrated religious. It is a warm garment, and protective and modest. And, in putting it on, you are put into a particular mindset. Further, it (should) protect you from people making sexual advances. It also puts faithful Catholics (and Orthodox) into a mental state of being reminded to pray.
That the FSSP uniform is just secular clerical dress is a let down for some, but it also is a reminder: they are, first and formost, priests, deacons, seminarians and minor clerics.