Too little tongue weight on trailer results in stability issues
Too much tongue weight is just hard on certain components.
The 10% minimum number is for an average trailer - any less and you get sway.
But a boat trailer, especially with outboard engined boat, often have the wheels set way back on the trailer. And as a result, you can get away with less tongue weight - say 7%. Of course, having less weight on tongue causes other issues - e.g. not being able to pull the boat up the ramp.
Aside: We reset the boat trailer wheels much further back to get tongue weight into 20% range, added a weight distribution hitch to Ranger. The result was we could walk the bow rider up the ramp, much to surprise/envy of many in the crowd.
Historically, RV and goose neck trailers had recommendation of 25% pin weight (Big rigs put 50% on pin). But that would limit towing capacity on current trucks, so it has been "relaxed" to 10-15%.