Decentralized service
Instead of just approving or rejecting a proposal, masternodes also form a service layer that provides various services. The reason that masternodes provide services is that the more services they provide, the more feature-rich the network becomes, thus increasing users and transactions, which increases prices for Dash currency and the block reward also gets high, therefore helping masternodes earn more profit.
Masternodes provide services such as PrivateSend (a coin-mixing service that provides anonymity), InstantSend (a service that provides almost instant transactions), DAPI (a service that provides a decentralized API so that users don't need to run a node), and so on.
At a given time, only 10 masternodes are selected. The selection algorithm uses the current block hash to select the masternodes. Then, we request a service from them. The response that's received from the majority of nodes is said to be the correct one. This is how consensus is achieved for services provided by the masternodes.
The proof-of-service consensus protocol is used to make sure that the masternodes are online, are responding, and have their blockchain up-to-date.