what is the use of gas ? It is also only for trading ?
As far as I know, gas are fees in Ethereum. The more gas you pay, the faster your transaction gets mined. Not sure what the role of gas will be once Ethereum goes Proof of Stake tho.
Example of gas usage: In the ICO's, rich whales used tons of gas to get their transactions go faster than the rest and get massive stakes on ICO's, so a lot of non rich people weren't able to buy the most hyped ICO's.