I am aware that there is a lot of documentation on ERC20. However because of all the hype about new tokens it is getting hard to find good material that isn't about ICOs.
I am wondering why are there so many ERC20 based tokens? What is the advantage of using a custom token? To the best of my understanding, the ERC20 tokens are validated and mined by Ethereum full nodes.
Can't all systems built using an ERC20 token just use ETH as a currency for transactions?