Because now deploys are immutable, you can’t push changes to a running instance - you just push a new instance, which means you get a new URL. But doesn’t that mean your URL is always changing? How do you share a link to your application? By using now alias to point a custom name to a specific deployment.
now alias <url> <new_name>
new_name: can be anything, such as: "demo-site"
url: is the now url that deployed.
But when you updated the site, you need to re-point the alias to new url.