Why and unlocked iphone ?
Basically you can insert whatever SIM card in your phone (when its unlocked) and select the subscription that better fits your needs than the ones proposed to you by your network career or for example, unlocking your iPhone means you can travel and change you SIM card with a SIM card of the country you are staying in.
Unlocked and contract-free iPhones are now available from the Apple stores but the price of your iPhone will be more expensive than when bought with a subscription at your ‘favorite’ network career.
The jailbreak is about the iOS. No more iOS upgrade for you but then you can download apps that you won’t find on the AppStore.
For your info, there about more than 700k apps (Dec 2012) available to your iPhone on the AppStore, so you should find what you are looking for. If you don’t check online and you’ll find lots of guidelines to follow to free you from Apple and jailbreak your iPhone.
Now it’s up to you, locked-unlocked or jailbreak!