Bike porn

I would say it's to optimise your BB height & position. Set the BB up where you need it and leave it there.

(thinking it could help riders with uncommon femur length and for different crank lengths)

Looks like 8-10mm of movement, so quite a bit.

You can equally use them to alter crank height too. A lot of SS offroaders like them. If you're running long crank arms, its nice to be able to increase your clearance slightly.

I see, that makes sense. Easier than getting a custom frame.