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).