Why do you need to plate or sputter additional material on the reed switch contacts?

The nickel and iron are relatively soft.  When you switch voltage and current  across the contacts some of the metal will melt and transfer to the other reed contact.  When you switch often enough, a sufficient amount of  metal will transfer and sticking will occur. Plating and/or sputtering a harder metal like rhodium or ruthenium will dramatically reduce the amount metal that is transferred and therefore directly increasing the life time or number of cycles before sticking will occur.