Why can I make a wallet backup on a microSD card and also write down my 24 recovery words?

The information stored on the microSD card and encoded in the 24 recovery words is identical. 
In both cases you have a complete wallet backup which you can use to restore your wallet, unless you are also using an optional passphrase with your wallet. 

Both options have advantages and disadvantages.

MicroSD card:

  • Advantage: Easy & quick backup and recovery on BitBox02
  • Disadvantage: If you want to import the wallet in a third-party wallet you need to use the backup recovery tool

24 recovery words:

  • Advantage: Common standard which is accepted by all major wallet which makes it easy to import the wallet.
  • Disadvantage: Manually writing down and checking the recovery words can be uncomfortable, slow and error-prone. 

In the end it's a personal decision which option you prefer or you just use both options.