A lot of if you need it comes down to if your state requires annual inspections. If so, best to keep it on. If no inspections then take it off and enjoy a smoother ride and better articulation off road.
What exactly does the track bar do? If I don't have a sway bar should I keep or get rid of track bar?
A lot of people drive without either on, though there is a universally accepted fact that if you drive on paved roads, you need a swaybar.
The Track Bars purpose is to eliminate the axle from moving side to side underneath the vehicle. With coil springs, this is a requirement, not an option. A coil spring can freely move side to side, front to back, up and down without much issue. That is why on a coil sprung axle, you will find yourself a set of Control Arms (they run parallel to the Chassis) and a Track Bar (or Panhard Bar, runs perpendicular to the Chassis). These components all work together to keep the axle in one relative position, and since the coil springs have nearly an unlimited directional movement, they do not cause binding, so the axle is free to flex up and down on either side.
With leaf springs, their movement is very limited side to side, so they already keep the axle(s) tracking perfectly underneath the Jeep without issue. A Track Bar actually causes binding with the way it has to pivot, and with the way leaf springs articulate. A leaf springs articulation is parallel on both sides (the tire going up tucks in while the tire doing down swing out slightly), whereas a Track Bar only allows the axle to pivot out on one side. coil springs do not bind up, because like I said, they can flex in nearly any direction without issue, but leaf springs will bind.
The Track Bars were intentionally put in place to bind up the leaf springs and keep them from flexing. This can cause damaged to bushings as well as other components, can cause bump steering offroad, and can actually cause your Jeep to steer on its own if you happen to drive quickly through large dips in the road. They are more harm than good on a Jeep.
There are larger leaf sprung vehicles that have a Track Bar in the front, but those rigs are typically intended for towing large loads, of which the Track Bar will help out a bit in front end stability. Our Jeeps are not made to tow large loads, they are designed to go anywhere and everywhere, paved road ro not, so they aren't much use with leaf springs.