In a SPA, you want to change routes in the URL so you don’t break people’s expectations about being able to share deep links to specific pages, but those sites don’t really exist. Eg if your site is www.example.com and you have a SPA on there, and one of the pages is www.example.com/products, you want to still serve index.html from the root even though the /products path is not a real directory on your system. If you don’t serve for this URL then it’ll work fine locally but if someone hard-refreshes or shares the link, it won’t load.