Question: Why do priests wear a Roman collar?
Answer: The practice of priests wearing the Roman collar developed in the mid-nineteenth century as an alternative to wearing the cassock which in some places was seen as impractical or which was even outlawed by anti-Catholic legislation. Since its introduction, it has taken different forms, and if you visit different countries today, you will see variation in what priests wear.
The Code of Canon Law simply observes that priests wear “suitable ecclesiastical garb” according to the judgments of the national bishops conferences and local custom (no. 284). The US Bishops have instructed that outside of liturgical functions, a “black suit and Roman collar are the usual formal attire for priests. The use of the cassock is at the discretion of the cleric.” Priests who are members of religious communities are to follow the rules of their institute regarding the habit or other approved clothing.
In the end, whether a priest wears the Roman collar, a religious habit, or, as is the case in many European countries, a suit and tie (to distinguish them from Protestant clergy who wear a collar) the goal of their clerical clothing is to serve as a sign of their commitment to guide and to serve, and to also allow them to be truly public figures because they are ordained for the service of all.