When you label a post with a category you narrow down the subject matter for example, to things like photography, genealogy, or birding. If you are searching with a specific topic such as “tattered curtains”, “fog”, (such as the photo I have added above) the chore is greatly reduced.
In addition, it is required that you give the current location of the topic or photograph, again to assist in locating it. As the contents of a blog grow and expand, a search becomes more manageable if the blogger has added lots of tags and categories. This blog has a search engine awaiting your interests, but needs time to grow.