What Is The Need Of “custom Controller” In Visualforce ?

1)      Sharing setting is applied on standard object or extension by default, In case we don’t want to apply sharing setting in our code then Custom controller is only option.

2)      A custom controller is an Apex class that implements all of the logic for a page without leveraging a standard controller. Use custom controllers when you want your Visualforce page to run entirely in system mode, which does not enforce the permissions and field-level security of the current user.