What does it mean that an attribute of Allah is necessary? The 13, 14, or 20 necessary attributes are rationally necessary. Meaning, that anyone with a sound intellect should be able to come to the conclusion that the existence of Allah is necessary, without having access to Revelation. It is impossible for one to deny a rational judgement, unless one is delusional or a sophist, as these are based upon rational truths.
Importantly, Islamic theology is only concerned with rational judgements, and thus the theologians are concerned with the study of rational judgements in depth.
For background see: The Rational Rulings (Hukm Aqli).