Many vapers move on to a more advanced kit once they know the basics of vaping. Sub Ohm kits give you more options including altering the wattage/volts and a Direct to Lung (DTL) hit which many users find more satisfying.
The batteries (mods) are generally more powerful than that of Starter kits as they need to power low resistance coils. Sub Ohm vaping basically means using a coil that is below 1.0 Ohm. Some kits also require 18650 batteries that need to be supplied and sold separately.