Lenskarts free glasses for 20k steps, whats the catch ?

so i saw lenskart giving free eye glasses worth 2k for free,if we completes 20k step in a day. so whats the catch here?

i know it will cost me somewhere around 800 to get actual eye wear, but aside for that whats the catch? is there quality issue with the frame? or like we can only select limited glasses ? i alr have vc glasses, so are they comparable ?