The Avalanche C-Chain, for example, is an EVM-compatible chain. The public key is what you would use to receive a transaction from someone else, or identify yourself in the blockchain. The private key is what you would use to make a transaction, or sign something. This functions similarly to a username and password on any online service. However, having to remember or walk around with your private key would be quite inconvenient, and honestly, rather unsafe. This is why there are many wallet providers out there that can help you access your wallet through a more standard interface, with a regular password, etc.