Because Human is Prone to Errors

Ever wondered how a good software should act? From "human is prone to errors" this point of view there are already several rules you can follow to determine an user interface behavior.

The first one is validations. A good software should be able to handle all kind of inputs that are submitted by the user. If it was good, process it. If it was bad, give the user an error message and chances to correct it. There should be no sort of submission that could make the software crashed.

Secondly, editable. All data should be editable because this world changes. and moreover, human makes mistakes, so you have to give them chances to repair, correct, retry everything they have ever made.

Third, simplify everything. User interface should be as simple as possible. Compare to a processor chip, human are dumb creatures. They can't easily memorize more than 5 numbers at once, or what they have done in the past view days. There is a popular term for this called wysiwyg [what you see is what you get]. For a best result, see at how popular softwares are doing it, and follow their behaviors.

To conclude everything, those 3 basic points are the least necessity of a software. Lacking one or more will make it disastrous to be used. That's all, validation, editable, and simple.

For more analysis to user interface design I would recommend a book written by Joel Spolsky. It's pretty good and enjoyable. Reading it has been a pleasure for me and I would never forgot these words quoted from it:: PEOPLE DON'T READ!