Use "not necessarily" in the following situation:

• Something is usually true, or most people think it's true.
• However, sometimes it's not true, or there's no proof of it.
• You want to point this out and disagree with what other people think.

For example:

A: I can't afford a car. A good one would cost at least fifteen thousand dollars.

B: Not necessarily. I saw a car the other day on sale for ten thousand.

A: Drinking is bad for your health.

B: That's not necessarily true. Moderate drinking can be good for you.

When you say that something is "not necessarily" true, it means that it's not always true. In other words, it might be true, or it might not be.

"Not necessarily the case" means "not necessarily true".