you do know that the main purpose of a blockchain is the preventing of double spends, do you?
So in DASH, the masternodes secure the blockchain (at least for IX-transactions). But I think I read somewhere that every transaction will be IX once evolution is out.
=> Masternodes secure the DASH blockchain.
I am perfectly aware, thank you.
I forgot to mention that Masternodes lock the transactions against double spend
instantly (hence IX), so you don't need to wait for block confirmation by the miners. Masternodes do not write to block though.
So to resume, Masternodes permit instant transactions while the miners secure the blockchain.
By the way, Masternodes also permit anonymity. It's a 2-tier network. The first of it's class. Soon to be a 3-tier.