When you tow a trailer, if you are using a standard, rear mounted hitch on on your vehicle , the tongue weight of from the trailer is transferred to the tow vehicle's rear axle. This can pull the back end of your vehicle down because of the added weight and make the front end point up towards the sky. If you have upgraded suspension on your vehicle, this is especially true. A Weight Distribution Hitch helps prevent this from happening by transferring the weight at the rear of your vehicle (the weight being towed) to all the axles on your vehicle and your trailer. This will result in a smoother, more level ride while you're towing at your maximum capacity.
If you are using your vehicle to tow something large and tall, like an RV, you'll want to look at either adding some type of sway control to your rig or upgrading to a hitch that offers weight distribution and sway control all in one. Sway Bars can be added to any setup and how many you will need for your rig depends on your GVWR and the hitch you are using. Adding Sway Control to your setup will greatly reduce the constant steering adjustments you have to make.