Generally: If you plan to bring along a sleeping bag, your own bed linens (sheets, covers etc.) or similar you won't need linens. If you don't plan to bring anything along, you do need them. You are not allowed to sleep in our beds without any covers, linens or similar due to our hygienic routines.
The fee of 50,- per person is a one-time fee to cover our expenses. By separating this cost from the room itself it is possible for those who want to keep the price as low as possible to save this money by bringing their own, thus helping us saving our expenses.