For others to make use of your work, they need to be able to find it first. Community registries are like the yellow pages for software -- registering your software makes it easier for others to find it, particularly through the use of search engines such as Google. Community registries typically employ metadata to describe each software package. With metadata, search engines are able to get some idea of what the software is about, what problem it addresses, and what domain it is suited for. In turn, this helps improve the ranking of the software in the search results -- better metadata means better ranking.
Community registries come in many flavors. Choosing the one that is best suited for your needs can be tricky. Here are some things to think about:
Finally, ask a couple of colleagues which registries they would use if they were looking for software like yours.