Masternodes are another way to secure the network and keep the blockchain moving forward. You have POS, POW, and other methods this is no different and it is essentially a POS system on steroids. With POS anybody can enter the network and secure it by using their coins the more you have the better. Masternodes is the same thing except they have a set coin amount (generally large amount) and those nodes are meant to secure the network. The benefit of having masternodes is that the community isn't liable to the 51% attack like they are in POW but the downfall is definitely the rich get richer. Good system and I overall like it but I prefer POS myself.
Thank you