The WordPress Search Functionnality searches for everything on your site. If a user types a word or the entire phrase in a search box, they will get results from the entire website and that includes posts, pages, categories, tags, titles and content.
The role of the URI is to used to identify a resource. However, there isn't one specific way to use a URI. The most suitable type will depend on the resource and the context in which you want to access it.
You can use the function get_search_form(); to display the search form. This function is primarily used by themes which want to hardcode the search form into the sidebar and also by the search widget in WordPress.
In summary, WordPress searches are an important functionality and understanding how to use URI's will allow you to navigate through your site easier because you can change the query to something that is more user-friendly.