Hello Joshua,
As far as I am aware they are used as fillets between the wing and fuselage joints. The claimed advantages are their light weightcompared to other methods of fairing in the wing / fuselage joint.
They are not very common on this side of the pond, but they have been used for a very long time in the US. I tend to crack the leather in my boots and shoes, so I look on this technique with some doubt. Mind you many people have used them for a long time, so my fears are probably groundless. Let's face it, most of my models suffer crash damage, long before any cracks might appear!
Regards,
Andrew.