In this week's Parsha the Torah devotes almost an entire Perek discussing the clothing of the Kohanim and the Kohen Gadol. One could ask: Why is it necessary to discuss the clothing at such great length? The Sefer Hachinuch answers that the clothing is very important because it reminds the Kohanim that they are serving Hashem, and that the Kohanim must have total Yirat Shamayim and Kavanna while doing the Avoda. Similarly, we wear Tefilin, Kippot and Tzitziyot in order to remind us that we are Ovdei Hashem, no matter what occupations we are involved in.
The Me'am Loez says that these garments were so beautiful and special that when Achashverosh made his party, he wore the garments of the Kohen Gadol. The threads of the Begadim were very fine and expensive. When one wears undignified clothing in public, he impacts negatively in a number of areas. First of all, there is potential for a Chillul Hashem to occur, people might say "Oh! look at that Jew who wears such repulsive clothes!" In addition, there is a element of personal embarrassment, because people will think that the wearer doesn't care how he looks. The Kohen Gadol, of course, is the representative of Hashem and of His Avodah. Therefore, he must be dressed in a dignified manner, leading to a Kiddush Hashem and an added element of respect for the Kehuna. We Jews are also the representatives of Hashem and must dress and behave with dignity.