why we will get dependents errors during exporting an application why not precedent error??

If you move an object into a new environment that is dependent on another item that doesn't exist then it will fail, you can't reference something that doesn't exist in the target environment.

If a precedent isn't in the new environment then nothing fails as nothing in the new environment is trying to reference something that isn't in that environment.

I'm a little confused by the question though so I may be misunderstanding what you're asking. Why would you want precedent errors?

thank you  

to clarify why only dependent errors  occured.

We get missing precedent error/warning while exporting application if a required object is not included in the package and not already in the target environment.