Hi group

I'm currently buzzy trying to understand interfaces and why I should use them.

I've come across an example explaining interfaces but I don't see why I need the interface.

Here's the example in short:
I have an interface named:IncreaseSalary
I have 3 classes:
CSupervisor
CStaff
CManager

They all implement the same interface (IncreaseSalary) however the result of IncreaseSalary is different for all 3 classes, that is:
CSupervisor.IncreaseSalary raises the salary with 5 %
CStaff.IncreaseSalary raises the salary with 3 %
CManager.IncreaseSalary raises the salary with 2 %

This is code for the client:
Dim emp as IEmployee

Select Case UserInput
case "Manager"
Set emp = New Cmanager
case "Supervisor"
Set emp = New CSupervisor
case "Staff"
Set emp = New CStaff
End Select

'pay the employee
emp.IncreaseSalary

So far it's clear. What I don't get is why I need to use the interface. Can't I just create the Method's in the 3 classes? That way i do not need the interface?!?! What am I missing?

Please help me out.

Regards,

Sander