why we use @Master type?

From Microsoft Docs you are defining the type of the Master property, which allows you to access the properties of your MasterPage derived class.

Provides a way to create a strongly typed reference to the ASP.NET master page when the master page is accessed from the Master property.

As an example:

this.Master.SomePublicPropertyOfMaster = Value;

Specifying the @ MasterType directive with a type of MyMasterPage results in the following property definition in the code behind class:

public new MyMasterPage Master {
  get {
    return ({MyMasterPage})base.Master;
  }
}

This property definition is created by the BuildMiscClassMembers method of the TemplateControlCodeDomTreeGenerator class.