Why would I want to jailbreak my Apple iDevice?
It lets you do everything from customizing the look of your iDevice to installing third-party applications (such as titles that are not authorized and available in the AppStore) and customized ringtones on it.
Depending on how far you’re willing to go, you can do even more than that because jailbreaking even lets you unlock your phone so you can use it with a carrier other than the one from which you purchased it (if your iPhone is carrier locked).
Are there any risks ?
As far as I am concerned there are NO risks jailbreaking your iDevice. Anything you do can be undone by a full restoration of your device. Sometimes you may get errors while restoring, most of them are software related and can be fixed.
Some issues you can run into by using a jailbroken iDevice:
You won’t be able to update iOS over the air – from the settings menu.
If you install too many tweaks, your battery may drain faster
Some tweaks may not be compatible with your devices iOS and may crash your SpringBoard
Some (rare) packages from Cydia can crash your whole system forcing you to do a full restore from iTunes
Most of these issues have an easy fix. What you have to remember is to have all your personal data backed up in iCloud in case you have to restore.
As long as you have your data backed up your device will be always ok even if you have to restore. You won’t lose any important data.
Again, advantages: there are plenty.
I will list some of the most important stuff you can do with a jailbroken iDevice (iPhone / iPod / iPad or Apple TV):
install custom themes for your SpringBoard – customize the look and feel
custom ringtones – there are plenty free ringtones
free internet tethering –
custom system toggles in Notification Center – turn on or off 3G / WiFi / Bluetooth / etc
iFile – file system browser
FaceTime over 3G with 3GUnrestrictor
QuickReply for SMS