必要 and 重要: requirements and importance
Distinguish what a purpose requires from what has importance, and recognize that unnecessary does not mean unimportant.
What a purpose requires
申し込みには身分証が必要です。 says that identification is required for the application. 必要, read ひつよう, marks a condition needed for that procedure.
必要な書類を先に確認しました。 says that someone first checked which documents were needed. 必要な defines the relevant documents; it does not establish that all were prepared.
Something with significance or impact
この説明は重要なので、よく聞いてください。 asks someone to listen carefully because the explanation is important. 重要, read じゅうよう, evaluates significance or impact.
重要な点に線を引きました。 says that someone underlined important points. They need not all be indispensable procedural requirements.
Need and value are separate judgments
この本は重要な資料ですが、今日の作業には必要ありません。 says that the book is important reference material but unnecessary for today's task. Value and present need are separate.
必要ありません says something is not needed for a purpose; 重要ではありません evaluates it as not important. Distinguish the meanings when declining an item.
Does 今日の作業には必要ありません mean the material has no value?
Show the answer and why
No. It limits the lack of need to today's task. The material's importance can be judged separately.