SharePoint uses the file system to store data files created by the indexing process. Over time, the index files grow due the indexing process. I recently ran across a case, where a disk filled up due to the continuous growth of the SharePoint index files. Needless to say, SharePoint's Search feature was not happy when the disk was full, and was unable to index content. What wasn't clear to the customer, was the fact that they did not have a healthy crawl schedule. A healthly crawl schedule should include both incremental and full crawls. This is because only full crawls clean up the index files created by the indexing process. Without full crawls, the disk your indexes are stored on will eventually fill up.