Full Idea
Anselm's second proof works, because he sees that necessary existence (or the impossibility of non-existence) really is a perfection. This is because a perfection requires no dependence or limit or impediment.
Gist of Idea
A perfection must be independent and unlimited, and the necessary existence of Anselm's second proof gives this
Source
comment on Anselm (Proslogion [1090], Ch 3) by Norman Malcolm - Anselm's Argument Sect II
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.54
A Reaction
I have the usual problem, that it doesn't seem to follow that the perfect existence of something bestows a perfection. It may be necessary that 'for every large animal there exists a disease'. Satan may exist necessarily.