Those of you with toppers, caps, etc. on your pickups...

As some have said, they make a seal for your tailgate. It keeps salt dust out in the winter.

I have used a little silicone sealant around the rear of my cap as there is a small gap between bed and cap.
It seems to work for me. Quick and cheap

I have an A.R.E. shell on my Tacoma, and yes, some water gets in around the tailgate in heavy rain. I have put foam all around, and there is less water. Also dust gets in, when I drive in deep sand. But I don't think the water or the sand have anything to do with the shell. The tailgate is just not even close to water-tight.

There are after-market gasket or weather strip kits that you can buy. I don't know how well they work. If we have "water sensitive" cargo (such as the generator), I just load it into the truck bed in a big plastic tub to protect it.

And I like this shell -- the hatch locks, and the side windows open, enabling me to reach cargo from the sides. And the windows lock, too.

snugtop does not leak
it has a design to keep the drips coming in from the rear .
i am a big fan of locking canopies on pickups .
i used to buy suburbans .
i cannot beat the versatility that the canopy on a pickup bed provides .
when i get home from a camping or boating trip the dirty stuff is in my pickup bed .
i can use my shop vac or i often take some palmolive dishwash and a 9 inch x 3 inch soft bristle brush , hot water and copious amounts of rinse water and get her clean again .
on a few boating trips per year i sleep under my canopy .
i want it clean in there .
other times i haul tree prunings to the dump .
easy to clean
cannot do that with a suburban
plus i have a duramax allison
cannot do that with a suburban

Your Leer dealer might carry the rubber seal for the tailgate. Goes on the bed not the tailgate. It works good across the bottom and most of the way up the sides on my 2011. I needed to stop the water where the rear door bottom seal, adjustment to the door ltches, and the center brake light was leaking. I built up the bottom of the window/door gutter in the corners where the flat rubber pieces are riveted to the cap. Sealed the top of the rubber and forced the water run off in toward the center edge of the gutter channel instead of the outside edge where it would run into the gap of the tailgate.

My Leer that I have had since 2000 does not leak water around the tailgate, but it does get dust inside when I travel dirt roads.

Never noticed any real water intrusions. But then again it is an open bed with a rudimentary cover on it, so a little water is no big deal to me.