Smartforms are nothing but function modules, therefore they need an interface in the form of import/export parameters to interact (to pass data in and out of the function module) with the driver program.
Program lines are subroutines of the function module in the smartforms. Like any other subroutines they also need importing exporting parameters because the variables used in smartforms are not global and they need to be passed on to the subroutines to identify them.
Regards,
Ashish