There are many parts on a vehicle that benefit from more ground clearance. Bigger tires give more clearance everywhere for sure, but you need fender/frame/suspension/steering clearance to run larger tires, and that requires some sort of lift, or extensive body mods.
To the OP:
A suspension lift will lift the frame, gas tank, bumpers, etc upwards as well as the frame.
A body lift will only lift the body above the tires and it won't raise the frame gas tank bumpers, etc...