They aren’t really interchangeable although they could both express necessity.
“Need” as a main verb expresses a desire or necessity. E.g. I need you to wait.
As an auxiliary, in the US, it’s almost never used and if it is, it’s more common in the negative. For example, You need not (it’s not necessary to) wait.
In the affirmative, “ I need hardly (it’s not necessary to) remind you to wait.”