What is the diference between MAMP & MAMP PRO?

The main advantage of MAMP Pro is that it simplifies the process of setting up virtual hosts. There's no need to edit the hosts file, which is a hidden system file. Nor do you need to define the virtual host in an Apache conf file. If you do a lot of development with PHP, the cost is probably worthwhile.

If I remember correctly, it also makes it easier to change certain PHP and Apache configuration settings. I have had MAMP Pro for many years, and I don't regret the cost. However, I tend not to use it very often because I create training materials, and I assume that most people use either the free version of MAMP or XAMPP/WampServer.