Why add file extension in route if you don’t serve file

I’ve seen some websites that add file extension to route, but they don’t serve html file from server , but rather compilled template. Of course I’m aware that result of compilling template is html, but I mean that you return it from the controller inside response object, rather than return file itself. I’m wondering if there is benefit in doing that and what it may be. Does it help when trying to download the webpage? If I remember corectly,there is .webp so that may not be it. I tried to google it, but with no satisfactory result. Thanks in advance.