necessary本身作名词是必需品的意思。
necessary:adj.必需的;必要的;必然的;无法避免的
No previous experience is necessary for this job.
这一工作无需相关的经验。
A radical overhaul of the tax system is necessary.
有必要彻底改革税制。
Some adjustment of the lens may be necessary.
可能需要调整一下镜头。