They're absolutely mandatory for when using mesh discs like Festool Net, Cubitron, etc, or you'll destroy the velcro hooks real fast, can't stress that enough.
I recently did some butterfly inlays, they stuck out 4mm above the timber, so using a mesh disc I used the thick interface pad to round over all the edges as it deforms over the inlays beautifully without tearing the disc or wearing any one point too much.
For the rest of the project I used the thin interface pad to sand the flat faces and sides with the mesh discs.
When I need to swap to a standard disc for a bit of roughing (low grits) I'll use the thin interface pad then too as I sometimes need to dwell on one spot or hold an angle to sand.