Why do i need the anonymous functions if i am sure $ alisa is used for jQuery?

Why do i need the anonymous functions if i am sure $ alisa is used for jQuery?

Hello, i am new to jQuery and i have seen some this practice in writing classes by jquery:

(function ($, app)
{
      app.Method = function () { ... }
})(jQuery, this.app = this.app || {});

i understand this anonymous function, and i understand the problem of $ alias. my question is this:
if i am sure that $ is used for only jQuery in website, why do i need this practice anymore?