Butane and propane are part of the light fraction that remains with naphtha in a simple atmospheric column.
A naphtha stabilizer is also known as a debutanizer - a depropanizer is similar - they are both side-stripping columns, or additional distillation columns operated in parallel with the main ADU column.
They remove the light fractions, which stabilize the raw naphtha, and produce lighter fractions which have other uses, like butane and propane, which can either be independently marketed, or blended for other uses, or in the worst case, used as fuel gas for the main or auxiliary heater.
De-bottlenecking is an optimization task over the whole unit, from feedstock in through product out.
Storage is always a problem, and its optimization varies over how the refinery is suppled, and how product is distributed. For truck or rail based supply/off-load, tankage is critical. It is expensive, takes up a lot of real-estate, and is expensive to maintain and permit. Too much is a potential liability and margin hit, which can sometimes be alleviated through renting out excess capacity. Too little is fatal - you impact stable, steady-state operations, on either side, input, or output.
Functional (or added) side-stripping capability increases the NCI by a bit - maybe another additive 1 to 1.5 depending on how it is implemented.
A very basic topping unit won't have any side-stripping capability, other than maybe an AGO/diesel reboiler. A unit that has side-stripping for the naphtha portion is higher up the food-chain, and a unit that has reformer capability is know as a hydroskimmer. This is the stage just short of full conversion capability, that extends into cracking and complex reforming.
