The external appearance of a person, and especially his clothes, is a thing of importance in the eyes of the Torah, contrary to what one might think. A good example can be seen in the very fact that the Torah devotes much attention to the clothing of the Kohanim, priests, and details the laws dependent on with great length. A Kohen, prepares and adjusts himself prior to his holy work by wearing clothes that were dictated to him precisely from Sinai, down to the very last detail - from which clothes to wear, down to the fabric and color each outfit is made of. Clearly, if clothes were not a thing of importance, the Torah would not go down this resolution when commanding over the building of the Mikdash.
However, beyond every particular example, it seems that the importance of clothes stems from the fact that the source of the need of clothes, begins back in the days of Gan-Eden. Walking in the garden, Adam and his wife, do not feel the need of a garment or any coverage for the feeling of shame is still absent. In an Ideal reality, prior to the sin of Eitz Hada’at, there is no need to be hidden, and the body can be fully exposed, just as Hashem walks in the garden without any cover. It is only after the sin, which the need of garment arises, after Adam deteriorates from the ideal reality of Gan-Eden. Hence, the need of garment is described as a phenomenon which relates to the inferior reality, and is needed only in the wake of the sin. The purpose of clothing is to hide and cover, to put a screen between people, and between man and God, screen which was not needed in an ideal world.
Following this primordial description of the source of the clothes, let us get back to the command over the clothes of the Kohanim. Viewing the garment as a phenomenon which stems from the imperfect reality after the sin, this lengthy command seems a bit odd. If the entire purpose of the garment is to hide and cover, why is there a need for designated clothes for the Temple work, the Avodah of Beit-Hamikdash! Enough if the Kohen would dress with proper modesty, fulfilling the Mitzvah of"ולא תעלה במעלת על מזבחי אשר לא תגלה ערותך עליו" (שמות כ כב), thus fulfilling the role of the garment. Why should we have white and gold garments, a bonnet and a jacket, for the sole purpose of covering those parts that should be covered?
It seems that when we come to speak about the clothes of the Koahnim, the function of the clothes elevates to a different level of functionality. If after the sin Adam and his wife are rushing to wear clothes in order to hide and cover, the Kohen wears clothes in order to reveal and uncover. The garment, as revealed in the Mikdash, can not only hide the wearer, but also reveal the hidden within.
There are multiple examples for this principle. Yaakov, the ultimate pure person, Tam, shows that there is little of Eisav in him as well, by wearing his brother’s hairy clothes. His hidden nature is revealed as soon as he is willing to wear different clothes. The Prophet Elisha wears the mantle of Eliyahu, Elijah, after his disappearance. It is only then that Eliyahu’s Spirit rested on him. The garment reveals the inner and hidden essence. But beyond every example, let us just look around us and see that just as their faces are different their clothes are different, and each person presents and expresses itself in a different manner and special set of clothes.
The same applies to the garments of the Kohanim. These clothes too were designed to reveal, more then to hide. The content which the clothes express is clearly defined by only two words: "ועשית בגדי-קדש לאהרן אחיך לכבוד ולתפארת" (שמות כח ב). On the one hand, 'Kavod', honor, towards the people whom are standing outside the Kodesh, and see the clothes appearance, and on the other hand, 'Tiferet', glory, towards the Kohen himself, even without anyone else to see. Through the clothes, the Kohen express his thoughts and intentions to the people gathered around the temple.
The Talmud says that a Kohen without his raiments, Bigdei Kehuna, is a stranger in regards to the holy, the Kodesh:
"אמר רבי אבהו אמר רבי יוחנן ומטו בה משמיה דרבי אלעזר בר' שמעון: 'וחגרת אותם אבנט' בזמן שבגדיהם עליהם כהונתם עליהם אין בגדיהם עליהם אין כהונתם עליהם והוו להו זרים ואמר מר זר ששימש במיתה" (סנהדרין פג ע"ב)
The Kohen who enters the Kodesh is required to express his inner world. The work at the Beit HaMikdash is not just an intimate rendezvous between the Kohen and his G-od, but it is supposed to be directed towards the profane reality. The advent of the Kohen, is designed to bring the Kodesh to the outside, and influence the entire reality, through the clothes which he is bound to wear.