Parshas Tetzaveh discusses the bigdei kehunah (Priests' clothing). The clothing that the Kohen Gadol wears is essentially Bigdei Malchus, clothing of royalty. He wears this clothing because he is the emissary of Hashem and represents Him in this world. In the Purim story when Achashveirosh shows off the wealth of his kingdom, the Gemara explains that he wore the clothing of the Kohen Gadol. He wanted to show that Hashem's people are lost and he is now the sole representation of G-d, therefore the people are obligated to serve only him. Mordechai puts on sackcloth, the opposite style of clothing, to show that we have work to do to get back to being representatives of Hashem. Only by taking off the royal garments are we able to once again don them and properly serve Hashem as His representatives.