My interpretations: First if we take 'Proof Of Stake', If you take staking - which is were you leave your digital wallet open to prove to the network you have that coin then you gain additional coins almost like interest. A masternode is like Super staking, you hold a higher number of the coin (Each master node currency will dictate amount required) and receive bigger interest (more of that coin back). Why pay you for doing this? With a masternode you are participating in saving information from the whole network/blockchain and in turn gaining a higher reward payment, where as staking you are only keeping a copy of partial network information and receive a lower reward.

Hope that makes sense.