As we grow old we get wiser!
So, in that sense I wanted to point out the use of the invisible field.
When we make for example a callback_before_insert (as the above example code states) we might need to exclude some fields from the add form but still want to pass some value to these fields that will be stored on the database.
The solution to this is actually quite simple.
We just use the fields function to state all of the fields that will take part on the insert process and this means also the fields that should not be showing up.
Then in order to "hide" the unwanted fields, hence the ones we want not to be displayed at the form and by hand pass them some value we mask them with the use of the field_type function with the use of the invisible type.
This gets quite handy and easy to use when we want for example to insert some extra data about the record that is saved on the database.
For example when we want to save the date/time of insertion, or if we need to know the user that inserted this record, or if we want to encrypt some sort of information like a password. The possibilites are just endless and the mecanism is there to help us out, straight out of the box!