The app automatically recognises the network on which the phone is running on and presents the user with a drop down list of available options (e.g. the O2 network in the UK has options for O2 - UK Prepaid, O2 - UK, TESCO and giffgaff). Choosing one of these options will change to Internet and MMS access point settings on the phone.
For those cases where the SIM / network is not recognised you can manually add Internet and MMS settings. In these cases you will need to specify the access point name, proxy address, proxy port, username, password and MMSC address. These settings are usually available from your operator's website.
The Network Setup app is particularly useful for those who frequently switch SIM cards (e.g. to get a better rate, or when travelling overseas). Without the app the only way to change the operator settings is to reset the phone, which requires you to restire all data and apps (a time consuming process).
Marketplace description:
The Network Setup application is for users who use more than one SIM card from different operators. The purpose of the Network Setup application is to maintain internet and MMS services after any SIM card change. When more than one operator network is available, the user is presented with a list to select their preferred operator.
Network Setup can be downloaded for free from the Nokia Collection in the Windows Phone Marketplace. The app is only available for Nokia Lumia devices, but equivalent apps are available from LG, Samsung and HTC for their own Windows Phone devices.