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In order for a web app to interact with the Ethereum blockchain (i.e. read blockchain data and/or send transactions to the network), it must connect to an Ethereum node.
Why use a library?
These libraries abstract away much of the complexity of interacting directly with an Ethereum node. They also provide utility functions (e.g. converting ETH to Gwei) so as a developer you can spend less time dealing with the intricacies of Ethereum clients and more time focused on the unique functionality of your application.
For more information on Ethereum libraries click here