If you switch on reply via same centre then when you reply to a text message, the message will only get through to the recipient if you are both on the same network (and using the same message centre).
In the olden days, you used to be able to use foreign message centres to basically send free text messages (before the networks realised they could make millions by charging a fortune for text messaging!). Now, message centres are set up to only accept and forward on text messages from customers on their network (e.g. I can't set my T-Mobile phone to send text messages through Orange's message centre).
Hope that helps?