As it has been explained to me, there are two key advantages to SXM having US Customs pre-clearance at PJIA.

One, because passengers traveling to the US will clear customs while still in SXM, airlines will be able to schedule flights to and from SXM from any US airport they choose, rather than only being able to fly from an international airport as is the case currently. So, passengers who might normally have to connect through JFK, or MIA or CLT or any other international airport, might find themselves with direct flights instead. It will, of course, depend on airlines determining that there is enough traffic to justify flights from certain domestic airports. There are bound to be some where it makes sense though.

Two, because airlines will not have to schedule their arriving flights to accommodate US Customs load factors, meaning coordinating their arrivals so that Customs does not have too many arriving passengers all getting in at the same time, airlines will be able to schedule flights at times that may be more convenient for everyone. This opens up the possibility of morning flights back to the US and so forth.

According to some government projections, SXM forecasts a five fold increase in the number of passengers arriving by air, as opposed to via cruise line, once pre clearance is established. I have no way to know how accurate this projection is but the economic impact of such an increase in visitors to the island is the reason that pre clearance is being pushed so heavily by some.