Indexes in Couchbase are a versatile tool for backend processing, enabling dynamic querying and selectivity, particularly useful in applications like inventory control systems. By leveraging a well-defined data model alongside Couchbase's indexing capabilities, users can create applications that perform decision tree-type aggregations and offer a high degree of query selectivity. Parameters such as key, startkey, and include_docs allow for flexible querying experiences. The text illustrates how an index, titled "catalogue," is built to emit data based on attributes like vendor, category, and discount, allowing targeted queries, such as retrieving items from a specific vendor or those with discounts. The system's ability to produce selective outputs and its integration with dynamic parameters enhances its functionality, demonstrated through examples from a clothing store inventory system, where specific queries return distinct sets of data, including entire documents when required.