The Lightning Network was created to optimize the Bitcoin blockchain, while still maintaining a level of security found on decentralized blockchains. Below you will find exactly what the Lightning Network offers.
Instant Payments: Enjoy near-instant payments with the Lightning Network. Although fast, network security is still enforced by blockchain smart contracts without creating on-chain transactions for individual payments.
Scalability: The Lightning Network is capable of billions of transactions per second across the network.
Low Cost: The Lightning Network allows for low costs because transactions and settling are done off-chain.
Cross Blockchain Transactions: Cross-chain atomic swaps occur off-chain instantly with the same level of security. As long as the chains can support the cryptographic hash function, you can make transactions across numerous blockchains without the need for a 3rd party intermediary.
If Bitcoin is intended to be used on a daily basis for transactions all around the world, then it needs to have scalability. Considering the Bitcoin blockchain is only capable of around 7 transactions per second (Visa is capable of around 21,000 tps), then there needs to be a way to improve the tps, hence the Lightning Network.
The main reason for the existence of the Lightning Network is to increase the transaction speed of Bitcoin by creating a second layer of multiple payment channels between parties and Bitcoin users.