Here's an example. A single-axle boat trailer with a 2,490-pound GVWR (trailer plus cargo) rolls on ST175/80R13C tires with a max trailer tire PSI of 50 pounds. At 50 psi, each tire is rated to carry 1,360 pounds, or a combined 2,720 pounds for the pair of tires. If the tire pressure is reduced to 40 psi, the individual tire-load rating drops to 1,190 pounds (2,380 pounds combined), or 110 pounds less than the trailer capacity. If you are in the habit of airing those trailer tires to just 35 pounds (your truck tires take 35 pounds, after all) the combined tire capacity drops to just 2,200 pounds. This could be a problem if the boat (including outboard, batteries, fuel and gear) plus the trailer weighs 2,400 pounds.