You need almost never use preLink. Viable cases for it are when you need to manipulate data in scope, but not the DOM, before link functions (also of other directives) are executed.
As jacob commented, you can always do that from a controller too, but sometimes it’s more appropriate to have the code in the directive itself.
There is an excellent article about how directives work where linking order is explained well to: http://www.jvandemo.com/the-nitty-gritty-of-compile-and-link-functions-inside-angularjs-directives/
If you need a good example of why pre-linking is sometimes necessary, I recommend you look at the code of angular directives themselves. For example https://github.com/angular/angular.js/blob/master/src/ng/directive/ngModel.js