Significance of pass value checkbox in function module

Hi all,

Here is a confusion.....While creating a custom function module,i have checked the update module or RFC call radio button going to its attributes.But in the importing parameter if i pass a structure,it is giving a syntax error.It is only allowing table parameter.But if i check the pass value checkbox,it is not givng any syntax error if i pass any structure in the importing parameter.Can anybody please explain me the significance of pass value checkbox?

Thanks in advance.......