I use mine to level the boat left or right that is port and starboard. In choppy seas I use them to push the bow down so that the sharp "V" in the bow cuts through the chop. The opposite of trimming your foot up, to raise the bow.
Used the same way as in a aircraft, and almost ALL aircraft have them. They "trim" the boat so it not only rides in the water better, but takes pressure off the captain. Especially very useful when a boat is loaded unevenly.(port/starboard). Once you have them (on a boat that can use them) you won't want to be without them. period.
Outboards and sterndrives have a lot of weight aft, which can create a higher-than-optimal angle of attack which can result in "porpoising" or the bow bouncing hard off the water. Add a little trim tab and it goes away.
Put them in the extreem down position for hole shot, then retract them when on plane.
Use them individually to balance an unbalanced boat.
Drop them a little in rough seas so the bow carves into the waves better
Lower them partially to allow the boat to plane at lower speeds
The getting wet part is why I'm thinking about installing them. Irecently boughta Cape Horn 21 (which is a great boat) and I know why the nickname "Wet Horn" is used. I have been playing with the motor tilt trim and have been finding ways to reduce the spray and that got me to thinking about trim tabs.