This is a verb, not an adjective. It's being conjugated differently because it's being used for third person. Él/ella necesita y yo necesito. "I [first person] need" is "yo necesito".
The subject-verb agreement for Él/Ella/Usted for (most) -ar verbs is to remove the ar suffix and replace it with an A. It's like how in English it's "I eat" even if I is singular and should supposedly be "eats", because that's the subject-verb agreement for I. I don't know if that made sense or not .-.
Because when a word is ended in a, it's feminine, so its used for women. If a word ends in o, its masculine, so its used for men. By the way, this answer is for mnfauch's question.