Additionally, why did the torah feel the need to discuss at great length all the priestly garments?

The Rambam (Hilchos Sanhedrin, 19;2) states that when a Cohen performs the service without his priestly garments it's considered as though a stranger performed it and the service is therefore invalid. However, why should it be considered as though a stranger performed the service when we know in fact that the Cohen performed it? Perhaps we could answer that the Cohen actually becomes a different person when he's not wearing his priestly garments. And why is that? Simply speaking, people feel different when they wear nice clothing (many people "feel like a million bucks" when they wear fancy clothing). This great feeling then effects the way the person acts and conducts themselves (no one acts the same when they're wearing a fancy suit vs. pajamas). It could be that the Rambam holds that the Cohanim could only be on a high enough level with their priestly garments in order to perform the service. Now, the main ability one needs to acquire in order to properly do teshuva is the ability to completely change oneself. When the Cohen would wear his priestly garments he would become a completely different person...It was only then that he was considered the Cohen. Were he to wear any other garments while performing the service he would be considered a stranger. Therefore, the Cohen would properly do teshuva by merely wearing the priestly garments because he would become a completely different person than he was before.

However, the service didn't merely serve as atonement for the Cohen--it atoned for the people as well! Therefore, why should the people also be forgiven for their sins if they haven't changed themselves?

Perhaps we could suggest that the people would learn from the ways of the Cohanim when wearing their priestly garments and through that they would come to do teshuva. Meaning, the actual wearing of the garments by the Cohanim wouldn't provide atonement for the people-the effect it would have on the them, however, would lead to atonement.

We could now understand why the Torah went through such great length discussing all the priestly garments--for one must first change their appearance in order to give themselves a better feeling in order to properly do teshuva. The only way to change the person you are is to change your clothes.

Summary: This week's parsha discusses all the garments worn by the Cohanim while they performed the service. The Gemara (Zevachim, 88b) relates that each of these garments atoned for a particular sin. However, how could one be atoned for sins by simply wearing particular clothes? The Rambam (Hilchos Sanhedrin, 19;2) says that a Cohen who performs the service without his priestly garments is considered a stranger. It seems clear from this that the Cohen is only considered a Cohen when he's wearing his priestly garments--otherwise he's just a regular person. Therefore, the Cohanim are able to be atoned for their sins through simply wearing their priestly garments because they're in effect changing themselves and becoming different people by wearing them.