This is a handy bit of grammar that means something like “it’s not always the case that ~.”
It seems like you use it when you want to point out to someone that a certain expectation is not certain. For example:
You might be planning to get married, but that doesn’t necessarily mean that your partner will feel the same, right?
On a side note, I used the Japanese dictionary goo to try and get a better idea of the specific usage. My Japanese is on the cusp of good enough to use straight Japanese-Japanese dictionaries. In the meantime, I learn quite a bit by trying to use them, :).