ERC-1155 enables you to issue many distinct NFT's within the same smart contract. This is very useful for related groups of NFT's, e.g. multiple editions or variations of the same art concept. Using ERC-1155 where applicable helps you reduce transaction costs (aka "gas fees") incurred by creating tokens when compared to using a separate ERC-721 for each token. With ERC-1155, you only have to submit the smart contract once instead of many times for each separate NFT!