1- The benefits of having a standard for all tokens are that applications that are built can talk to each other, all wallets know how to interact with ERC20 tokens, and this creates efficiency and takes away friction, thus promoting better progress in this space!
2- There is totalSupply, which tells you how many tokens are in circulation. There is balanceOf, which allows you to input the address of the account you want to know the balance of. There is transfer, which sends tokens from the sender of the message to the receiver. In addition:
approve: a token holder gives another address approval to transfer up to a certain amount of tokens
transferFrom: allows the receiver to TAKE funds from a certain address/account
allowance: provides the number of tokens that can be transferred from one address to another.