What are the white tubes included with the MK3S used for?
Its the ptfe tube that is part of the hotend assembly. That part is pre assembled on the kits. You can see it pictures here https://help.prusa3d.com/en/guide/5-e-axis-assembly_28536 in step 12. Its held in place by the black plastic clip (push inwards towards the metal to release the tube , allowing it to be removed. On the original e3d hotends sourced from e3d they come with a small clip that goes under the black locking collet to stop it accidentally going down during use. For the life of my I have no idea why prusa dont fit/supply these. I always fit them myself (one of the first things I printed, theres loads of them on thingiverse). As they help stop the ptfe tube getting loose and you dont want a gap.
One end is chamfered on the outside and that end goes in first so meets the metal. The side that sticks out/up should have a small chamfer on the inside to help guide filament into it.
Prusa's instructions for it are here https://help.prusa3d.com/en/guide/how-to-replace-a-ptfe-tube-mk3s-mk2-5s-mmu2s_21664/