You might recall from the movie The Ten Commandments how they portrayed Mount Sinai as being God's dwelling place. That is a true representation. I can remember the first time I saw that, I screwed up my face. I said, That can't be right. Yes, it was.
Moses was standing on the dwelling place of God. It was God's territory.
Now where does a priest serve? He serves where God lives. He serves at God's altar. Where was God living symbolically at that time? He was living at Mount Sinai. And where was God's altar? The whole mountain was God's altar. He not only had a dwelling place, He also had an altar. It was a mountain.
So here we have another setting apart. In this case it was an entire mountain to serve as God's dwelling place, and the altar at which the Israelites were to worship and to serve Him.