When you create a Functions App in Azure, you choose its name and Azure provides you the URL of the home page for the app.

You have this URL even your Function App doesn't contain any Http Trigger Functions and if you try to navigate to that url, you can see the following page:

If you want, you can disable the welcome page using the AzureWebJobsDisableHomepage setting in the function app configuration.
You disable the welcome page setting the Function App configuration as in the following picture:

With this configuration, if someone tries to browse the previous url, he gets an HTTP Status code 204 (no content).