Download the trainer, set it to run in Administrator mode, then open it and make sure the SMP is set to BigPond/Trymedia. Open the game with the trainer open, load your character/start a new one and then make whatever adjustments you want. Worked for me.
As for "why use it?" Well, sometimes I just want a sandbox pirate game and so far this has the best engine/mechanics/controls that I've been able to find; it's easy to understand and fun to play, but I don't always want to have the whole "storyline" quest hanging in the background and being a downer at the ending if I didn't finish it. Also, while a challenge can be fun sometimes, other times you just want to roll out a character who's epic from the start and rampage your way across the Caribbean. Plus, I think the trainer can be used to make things harder or randomize things like town ownership to give more variety.
Edit: Also, at least some of the quests/achievements have a chance of being impossible to complete due to RNG (there is aparently nothing keeping the game from creating a map with no beautiful daughters for instance) and the trainer can help with some of that.
Post edited April 22, 2020 by Childe_Roland