Many clients have more than one rate relevant to an employee. If these rates are added into the employee profile as an addition they appear on every payslip as a zero value unless they are manually deleted which is a big job on a large payroll. The ability to be able to add additional rates to an employee that only appear if you process hours against them would be invaluable.
Alternatively the option to not show "zero lines" on a payslip would work as well