The ERC721 token is also called NFT (Non-fungible Token) and NFT is a non-replaceable token. While each ERC20 token has the same value, the ERC721 token has a unique value for each token.
In other words, it is impossible for the NFT to replace one token with another.
ERC20 is issued for functions similar to currency or points. On the other hand, ERC721 is used in cases where each token has a unique value, such as a game item.