calling a Function module without using service call in Wedbynpro/ABAP

Hello Gurus,

I have been working with a Webdynpro development.

In my Webdynpro application development, I am using a Function module to get the PO numbers based on the time frame of PO's.

Here i have not used Service Call to call my Function module. I directly called the FM and my FM is giving me the PO no's.

When should i need to use Service call option of Webdynpro component? In this particular case dont i need to use service call?

What is the difference between calling a FM directly and through Service call in Webdynpro/ABAP ?

Thanks in advance.

Regards

Dev