必要 and 必要性 usages

Question

What's the difference between 必要 and 必要性 and in what situation should I use which

Answer 1

First of all, 必要 is primarily a na-adjective rather than a noun. It means "necessary".

• ✅必要な量　❌必要性な量
  necessary amount
• ✅それは必要だ。 ❌それは必要性だ。
  That is necessary.
• ✅必要になった。 ❌必要性になった。
  It became necessary.

If you have really used 必要 only as a pure noun, this is the most important difference you have to remember. (And I think this is why your question looked confusing and got downvotes. Your question almost looks to me like "What's the difference between necessary and necessity?", which is too elementary.)

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That being said, there are some cases where 必要 is used as a noun. In some cases, 必要 and 必要性 are interchangeable, but in some cases, they are not:

• ✅読む必要がある　✅読む必要性がある
• ✅読む必要がない　✅読む必要性がない
• ✅必要に応じて読む　❓必要性に応じて読む
• ✅読む必要を感じる　✅読む必要性を感じる
• ✅必要に駆られる　✅必要性に駆られる
• ❌必要を検討する　✅必要性を検討する
• ❌読む必要が高い　✅読む必要性が高い
• ❌読む必要が乏しい　✅読む必要性が乏しい
• ❌ワクチンの必要について　✅ワクチンの必要性について

It's hard to generalize and I may be wrong, but I feel:

• When the focus is the degree of necessity (高い/低い/etc), 必要性 must be used, whereas 必要 and 必要性 are usually interchangeable when the focus is the existence of necessity (ある/ない/感じる/etc).
• 必要 follows a verb, but it tends not to be linked to a noun using の.
• 必要性 sounds a little more formal or academic.

Answer 2

Dictionaries say 必要性 means そのものが、どれだけ必要であるかの度合い. That is "the degree of necessity". For example, we can say 日本語を勉強するうえで、この教科書の必要性はとても高い. 必要 can't be replaced with 必要性 in this sentence.