What do ladder bars do ?

Brooks James said
What exactly do they do ?
Current set up 8 3/4 axle sure grip 3.91
70 Dodge Dart, Mopar SS Springs, adjustable pinion snubber
What more will they do Over my current set up ?
Just want a basic set up, bolt on , no crossmember, no coil Springs

They reduce the axle from coiling up under heavy acceleration which reduces wheel hop and loss of traction.