A thing exists necessarily because it is it’s nature to exist, and if it is a things nature to exist it cannot possibly not exist. If a thing can only possibly exist rather than necessarily exist, then it follows necessarily that it is not it’s nature to exist since it does not naturally-exist , and we can only make sense of it’s existence if something is making it exist. Otherwise what reason is there for it’s existence or even the possibility of it’s existence? If not a necessary reality, then what you have is a being that arbitrarily-exists ; existing not because of it’s own nature or because of the nature of something else, but for no reason at all. This is a brute fact, and i reject it because it doesn’t make any intelligible sense.