Users are useful to a currency because more users means more stability and speed. Specifically, more value stored and more value traded. (for the network as a whole)

More miners means more transactions can be secured more quickly at a more competitive cost. (to be competitive against other networks)

Proof-of-Work (POW) creates "truth" by hashing with real energy (electricity) for subversion resistance from money printing. Using printed money to buy hashing power supports the speed of "truth" and the strength of truth.

Proof-of-stake (POS) can be bought with printed money without necessitating supporting the strength and speed of the creation of "truth".

Here the original papers on the value of Proof of Work, cited as references in the Bitcoin white paper:

• http://www.weidai.com/bmoney.txt
  *http://www.hashcash.org/papers/hashcash.pdf