Why do We Wear a Tonia?

According to the “Dictionary of Church Terms” by Fr. Tadros Yacoub Malaty:

Tony:A white tunic worn by all monks and priests while celebrating the Holy Mass. It is white because it represents the joyous angels who are serving God in purity and holiness.

So the purpose of wearing the tunic/vestments is to give glory and honor to God. It also distinguishes the various clergy and laity whose liturgical roles differ. Moreover, we can further look to the command of God given to Moses as recorded in opening verses of Exodus 28 to see evidence of God's desire for special vesture for His priests and other liturgical ministers.