what is the best practice to choose whether to extend your web application?
When we need to make our web Application as a public website for public users and keep one for portal admins, and when have the same web application for both.
It totally depends on the requirement:
1)if you require your application to have different authentication type.
2)Or you have diffent target user for your application.
Basically by extending WebApplication you create a new Site IIS Site with same application with same/different configuration for WebApplication.