Why Do we Need "Actions" in Module Development

Hi - I have read various pieces on what actions are, however, no where have I seen a piece on why we need actions.

From the pieces I have read, it appears that actions are required simply because the available function does not do what we need it to do. For example, the form_declaration() function sends form data back to the index page, so, if you need it to go somewhere else, you need to intercept that and send it to the correct location. You accomplish that using an action. Or at least, this is how the tutorial below explains it:

http://ee-recipes.com/articles/details/adding-a-form

However, since I come from Codeigniter / PHP background I look at this, and think, this is bizaar. The form_declaration() is so broken as a function, that we have to “fix it” with another function …. no …. I must be wrong.

Yet - no where can I find a cogent explanation of WHY we need actions .. and while I’m at it, is there any sure fire way for a newb also to know when you need an action?

Please some one put me out of my misery and explain, the why and when of using actions .

Many thanks !!