How can 24 words allow a user to recover a public and private key? I understand that the Ledger Nano S allows one to create a 24 word recovery phrase.

A) Why is it better to have users keep a 24 word recovery phase as backup than to keep the public and private key as backup?

B) How does Ledger decode the recovery phrase into a public and private key?
A) For each hierarchical deterministic account path ~2 (if detached) to ~4 (if not detached) billion private keys and associated public addresses can be synthesized.  If BIP 44 technology is applied to HD wallets.  The same wallet can manage multiple cryptocurrencies, see this table. 

B) Take a look here that concisely shows how to recover private keys and addresses for multiple cryptocurrencies using the BIP 39 seed words found here