Needful, a.
1. Full of need; in need or want; needy; distressing. [Archaic] Chaucer. The needful time of trouble. Bk. of Com. Prayer.
2. Necessary for supply or relief; requisite. All things needful for defense abound. Dryden. -- Need"ful*ly, adv. -- Need"ful*ness, n.
Shades of meaning
Fernald has no entry of its own for needful; he lists it under necessary.
Where he writes about it:
Needed and needful are more concrete than necessary, and respect an end to be attained; we speak of a necessary inference; necessary food is what one can not live without, while needful food is that without which he can not enjoy comfort, health, and strength. — from necessary