Many apps require access to the filesystem for accessing databases, reading content, and many other reasons.
There are two main areas for storing files: internal storage and external storage. Making use of the internal storage or app sandbox is very easy:
string sandbox = FilesDir.AbsolutePath;
string file = Path.Combine(sandbox, "myFile.txt"); bool exists = File.Exists(file); File.WriteAllText(file, "this is my value"); string value = File.ReadAllText(file);