Yes, the benefit is exactly as stated above. If you use a spacer that fits on top of the strut assembly (top-hat), the entire assembly is then longer and can therefore possibly overextend the designed angle of the upper control arm ball joint at full droop, according to the popular belief. This is the reason most top-hat style kits come with new control arms. The benefit of this style of lift is that the spring doesn’t have any added pre-load, so it won’t ride much stiffer.
With pre-load spacers, the overall length of the strut assembly is unchanged, but the increase in spring pre-load causes the truck not to have as much static sag. It just rides higher in the original “range of suspension. The benefit is that you theoretically can’t overextend the ball joint angles any further than stock at max droop. The angles are just a little steeper at normal ride height, which isn’t a big deal. The downside is that if you don’t get shocks that are valved for the pre-load, it will give the feel of a stiffer ride.
Some kits out there use both for added lift height, like the 3.5” kits from R/L or R/C. I had a 2004 F-150 FX4 once that I combined pre-load and top-hat spacers on to get 3-3.5” of lift on. It looked great but the ride was very stiff...