I am using a HTML Boilerplate and I see a file called site.webmanifest. I have searched information on the Internet and I don't understand the use of it.
Is it a mandatory file in the development of web pages? When and why it is used? How do I know when I have to use it?
Please refer to documentation here.
It is not a necessary file. You need to use it or you should use it when you are designing a web app that can run offline. Examples are whatsapp web, todoist webapp, amazon webapp, etc.
Web app manifests are part of a collection of web technologies called progressive web apps (PWAs), which are websites that can be installed to a device’s homescreen without an app store. Unlike regular web apps with simple homescreen links or bookmarks, PWAs can be downloaded in advance and can work offline, as well as use regular Web APIs.
The web app manifest provides information about a web application in a JSON text file, necessary for the web app to be downloaded and be presented to the user similarly to a native app (e.g., be installed on the homescreen of a device, providing users with quicker access and a richer experience). PWA manifests include its name, author, icon(s), version, description, and list of all the necessary resources (among other things).