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Some clans put this as a requirement so they can get more forum active members. As members with low post counts usually are either new to the forums, or are inactive.

A high post count usually shows that you have experience with the forums.
Experience is a major requirement for most clans.
If you have another way to express the fact that you have experience in the forums, you should tell them to make an exception, disregarding your low post count.

Depends on each leader/clan, it's most likely to prove you are forum-active.