Unlocking iPhone 3G would mean that users can simply use SIM cards from different network carriers in their own country.

Sources:
Jailbreak 4.1
Jailbreak 4.2.1

In several countries, there is limitation on iPhone users to switch to another network service provider aside from AT&T, until and unless they unlock it.

Sources:
Jailbreak 4.3.1
Jailbreak 4.3

If they desire to get access to third party applications or use SIM card from any other network carrier, they need to perform iPhone 3G jailbreak.

Sources:
Jailbreak IOS 4.2
Jailbreak 4.2