Why setting HashTable's length to a Prime Number is a good practice?

Question

I was going through Eric Lippert\'s latest Blog post for Guidelines and rules for GetHashCode when i hit this para:

We could be even more clever here;

Answer 1

Because this produces a better hash function and reduces the number of possible collisions. This is explained in Choosing a good hashing function:

A basic requirement is that the function should provide a uniform distribution of hash values. A non-uniform distribution increases the number of collisions, and the cost of resolving them.

The distribution needs to be uniform only for table sizes s that occur in the application. In particular, if one uses dynamic resizing with exact doubling and halving of s, the hash function needs to be uniform only when s is a power of two. On the other hand, some hashing algorithms provide uniform hashes only when s is a prime number.