This is standard if you have backup wallet chosen in your settings. Alternatively, email backups will send a wallet.aes.json file to your specified email address. A few things to keep in mind:

• The aes part means your wallet is encrypted using your Blockchain login (and secondary password). If your password is changed then your newer wallet files can only be decrypted using the new passwords
• The json part means the decrypted aes is a readable json format which can be read by humans and imported by various programs
• Changing your password will change your decryption mnemonic.

The backup wallet.aes.json files are good for recovering old wallets but if your BTC has been moved from those addresses it does not recover your money.