Query parameters in a URL are not always present. In the context of websites, query parameters are often used for marketing analytics or for changing a variable on the web page. With website URLs, query parameters rarely change which page you're viewing, though they often will change the page's contents.
That said, query parameters can be used for anything the server chooses to use them for, just like the URL's path.
https://www.google.com/search?q=hello+worldhttps://www.google.com/search?q=hello+universeYou should see new search results for the query "hello universe". Google chose to use query parameters to represent the value of your search query. It makes sense - each search result page is essentially the same page as far as HTML structure and CSS styling are concerned - it's just showing you different results based on the search query.