I have transferred orphaned airline miles into hotel points, car rental points into airline miles, etc.

But, I don't actually know how does this work, in terms of the transferor?

For example, if I transfer 15000 Marriott points to WN, then is Marriott actually BUYING me the miles, albeit at some lesser sum than I can if buying miles on WN? In other words, does it cost Marriott cash to exchange the points into miles?