Jailbreaking opens up a whole lot of opportunity for cool things on your iPhone that you couldn't get without a jailbreak. However, with cool features usually comes more battery usage. For instance, you can get cool features like animated home screens or whatever, but those animations take juice to power.
An easy way to think of it is this; jailbreaking your iPhone allows for a lot of new stuff, but the new stuff needs new battery life usage.
After a JB always reboot, any hung processes that are not visible to the user will be closed. I had run field test mode on my phone and thought I had closed and lost 60% battery in about 2 hours once I rebooted I was fine the rest of the day.....
