The idea is your tank uses up those components over the week, getting deficient in them. Then on waterchange day there is enough "extra" to make up the difference and get you back to where you want to be. If the salt were set at perfect parameters, your tank levels would go down over time.
Reef crystals also has other stuff added, I think they call it "vitamins." This is supposed to be good for the tank but some people don't like it because they say it leaves a grunge in their mixing bucket after a while, and can cause some ammonia in the water. I haven't had any trouble with it, but I don't let my water sit around in big trash cans for long times, I just mix and use.