In a centralised exchange: Whenever a transaction of Crypto happens, a third party needs to be involved( most probably the Exchange itself ). All the cryptocurrency used is stored inside the parent wallet of the exchange itself. This makes the exchange extremely un-secure and prone to hacks. The owner of the exchange needs to invest significant amount of time and money in maintaining the security. Also since every transaction is executed by the files present in the exchange > the owner of the exchange needs to spend money on maintaining a good server infrastructure.

Eg: Binance,CEX.io etc.

In a Decentralised Exchange: Every transaction happens on the fly, directly between the wallet of the respective users. The underlying smart contract defines the entire transaction in a safe and secure manner. Since no cryptocurrency is stored on any centralised servers > No need to worry about security. Also the entire exchange runs on the browser of the user ( via. JS ). So, just any normal server is more than required to host the entire exchange and make it run in Super speed.

Eg: EtherDelta, Waves Dex etc.

Top 3 reasons to run a Decentralized Exchange:

• Very Less Expense running cost : Unlike a centralised exchange, you don’t need to spend a fortune on running a sophisticated server ( as everything runs on the customers browser ). No need to spend huge on security ( as all crypto are stored on the users wallet directly ). You Set it up & The exchange just keeps automatically under minimal server specifications.
• Very High Recurring revenue: Every time a transaction happens > You earn Crypto that gets transferred to your Wallet automatically. You can configure the commission percentage per transaction on your smart contract.
• Users love it:Since there is NO OTHER fee associated with each transaction ( except the minimal fee you setup ), users love to transact on decentralised exchanges. FYI: EtherDelta charges 0.03 % fee and handles 4 million transactions per day!

One more thing… You can charge a listing fee for each token you add in the exchange!