Only God is candidate for Necessary Being.
The Necessary Being is seen to exist by virtue of being necessary.
The first premise shows why this doesn't apply to other things or beings.
The second premise is an obvious fact, if seen. The only question is there a Necessary being.
Anselm argues when the greatest being is conceived, it's seen to be great to the extent of being necessary (level). So when you recall that it's greatness is at the necessary level and no other being has this quality, then it's known it exists. You can only imagine beings less great then it not existing, but if you understood necessary and it's greatness, you can't see God not existing. It's virtually impossible if you grasp both these aspects. But it has more implications.
Not only is God known to exist, but that it alone is eternal. Not only is it alone eternal, but it can't be repeated, and so can't beget nor is begotten. The reason being is that vastness implies it exists in all possible worlds while things come to be by definition as not necessary.
And there is only One necessary being possible and so there is nothing on par with it.