1. The Roman collar is a sign of priestly consecration to the Lord. As a wedding ring distinguishes husband and wife and symbolizes the union they enjoy, so the Roman collar identifies bishops and priests (and often deacons and seminarians) and manifests their proximity to the Divine Master by virtue of their free consent to the ordained ministry to which they have been (or may be) called.You can read the entire list over at Courageous Priest
2. By wearing clerical clothing and not possessing excess clothes, the priest demonstrates adherence to the Lord’s example of material poverty. The priest does not choose his clothes – the Church has, thanks to her accumulated wisdom over the past two millennia. Humble acceptance of the Church’s desire that the priest wear the Roman collar illustrates a healthy submission to authority and conformity to the will of Christ as expressed through his Church.
3. Church Law requires clerics to wear clerical clothing. We have cited above number 66 of the Directory for priests, which itself quotes canon 284.