MyEtherWallet, popularity knows as MEW, a a website where we can create an online wallet to store our crypts without downloading all or part of the blockchain.
The good thing about it is that you don't store your Ether on its servers, but simply allow you to create your own wallet and make transaction through its node.
Now, why use it?
The answer is that it is not necessary to have it, but it is advisable to avoid the risk of losing our assets if the exchange in which we keep our cryptocurrencies is hacked or ends up closing.
Also, as I said, by not storing your cryptocurrencies on their servers, as with other solutions, YOU HAVE COMPLETE CONTROL of your wallet, no matter what.
That is why it is considered one of the safest wallet that exists today.