A wallet is a tool to help you manage your cryptocurrency. Some, like Argent’s, can be used to help you access blockchain applications too (also called decentralised apps, or Dapps).
Why are wallets necessary?
In cryptocurrency your funds are held on a blockchain. Private keys control access to them. Wallets are a way to manage the private keys. You can either have a third party keep your private keys and manage the funds for you. Or you can use a non-custodial wallet and manage the private keys yourself, as with Argent’s wallet.