How rar packs and protects files?

I don't understand, what you want to know. The "salt" bytes are useless for decompression. They are put in front of the compressed data before encryption. For the same reason, they are useless after decryption, since they are not needed for the decompression.

The idea behind the "salt" bytes is to make the encrypted data look different. Like already said, it makes crypto-analysis a bit harder.