tweak readme

readme.md

@@ -7,7 +7,7 @@
 Noshell is a simple shim that executes `$XDG_CONFIG_HOME/shell` as your login shell.
 If you configure noshell system-wide, users can configure their login shell with
 a simple `ln -sT /path/to/zsh ~/.config/shell`, instead of requiring
-superuser permissions to modify `/etc/passwd` and `/etc/shells`.
+superuser permissions to modify `/etc/shells`.
 
 By default, noshell checks the following locations in the order:
 
@@ -23,3 +23,13 @@ programs.noshell.enable = true
 ```
 
 Noshell will be configured as the default shell to **ALL** users.
+
+## Why?
+
+Noshell enables some patterns in the NixOS ecosystem, such as:
+
+- Using a shell with a configuration wrapper. As the wrapper is a different program
+  that is not present in `/etc/shells`.
+- User-level configuration of the login shell with home-manager.
+
+It can be useful too outside of NixOS, for example to use a custom build of some shell.