Imports come with no underbody protection at all, UK cars have some applied before they leave the factory,
Pro's preserve the life of the car, drives out moisture and kills any little bits of rust before they take hold,
Cons, you need to clean the underside of the car before applying, so either pay somone to do it £500 plus for a full seam clean, dry, then application or the cheaper diy method of jacking up crwaling underneath with a wire brush and removing all the loose dirt and crap then apply new underseal.
I've used the waxoil spray kit which was a total night mare, loads of overspray on bits I didn't want covering ie manifold & exhaust and it's even messier than the stuff you brush on as it just keeps on coming out even when you've backed off the pressure
For the sake of 15-20 quid for a pot of underseal A spray can of underseal and a wire brush/paint brush maybe two hours i'd go diy.