You have to change springs, when fitting the dampers, as they take some of the load on the new dampers so I fitted some when the springs broke.
It's intended to compensate for heavy loads. As you know, I've recently done the kitchen and had several Ikea runs heavily loaded. When first loaded, the car was very low, but when I got home was lovely and level again, so obviously doing the job well. The car drove like the load wasn't there, except for braking of course.
So they certainly stiffen the back under load, but I can't say if it will help with pitching when towing. I say this, because they kind of pump up, as you travel along. The noseweight of your towed load would do the same, so they would stiffen. However any pitch which lifted weight off the tail, would immediately release the pressure and lose the effect. I would say if you need to do the springs anyway, go for it.