This is how PostgreSQL's official documentation defines Indexes, pretty simply and clearly — "Indexes are a common way to enhance database performance. An index allows the database server to find and retrieve specific rows much faster than it could do without an index. But indexes also add overhead to the database system as a whole, so they should be used sensibly."
The last word, "sensibly," in this definition, is the crux of this article. Indexes are good, and they enhance the performance of queries. This does not imply that we create Indexes for every query and every column. It is important to remember that while Indexes do enhance Performance, they do need to be maintained, and that is overhead.