Needful, necessary, requisite, indispensable, essential are comparable when meaning urgently required.
Needful carries the weakest suggestion of urgency, but it applies to something that is required to supply a want or to fulfill a need.
Necessary implies more pressing need or urgent constraint but, except where the compulsion of necessity in the sense of an inherent, logically compelling principle is suggested, the word need not connote that the thing so qualified cannot be done without but more often indicates it to be infinitely desirable rather than absolutely required; thus, tires are necessary to proper management of an automobile, but in a sufficient emergency one might drive without them.
Requisite differs from necessary chiefly in being applied to something that is specifically required by the nature of a thing, the end that is in view, or the purpose to be fulfilled; usually the adjective suggests an imposed requirement rather than an inner need and so suggests constraint from without or, often, from official sources.
Indispensable not only carries a stronger implication of urgency than the preceding terms, but it also distinctly implies that the thing so qualified cannot be done without, especially if the implied or expressed end is to be attained.
Essential (see also ESSENTIAL 2 INHERENT ) is often used in place of indispensable as implying no less urgency but as being less extravagant in its suggestion; it usually also implies inherent necessity from the point of view of what a thing is or must be by its very nature or end.