Why should I write a library rather than a helper?

[eluser]OES[/eluser]
I was told to think of it this way.

If you have one off functions then helpers do the trick like you have said. But lets say you building a app eg a Calander.

It would make sense to add all functions in one library for the calendar. You just have to look at CI Librarys. All functions are for the same thing.

Hope that makes sense!