You might find them useful.
Here are what I see as some reasons to get them, and reasons not to (though some of my reasons to get them might not be a good reason for you, and some of my reasons to not get them might not matter to you):
They're great if...
you want a stable 45-degree-angle surface on which to do incline work
you have the Firm Transfirmer workouts (or want to do incline step cardio): the risers make it possible to make a version of the Transfirmer (the other day, I used my slanted risers, my two step platforms---yes, I have two!--my High Step topper and an assortment of risers to make all the configurations necessary to do the "jiggle-free buns' workout).
you have the High Step topper and want more risers for it (the slanted risers stack together to make the height of 3 regular risers, and they are more stable than stacking 3 risers).
They're not really necessary if...
having only a 45 degree option as well as flat bench isn't that worth it to you
you don't intend to use them for anything but making an incline bench
you don't have the High Step topper, and don't intend to get it