It tends to rain a lot up here and with a level trailer the slide can hold water in a heavy rain that can make it through the seals. Especially if there is a driving wind.
I installed a Carefree by myself in an hour or so and have not had any leak issues since. Also, there is very little debris collecting on the slide top.
I think I paid about $500 for the topper and the installation was free.
My second slide is covered by the trailer awning so I only needed the one for the drivers side of the TT.
Highly recommend getting one installed.