What is Arbitrum? Arbitrum is a technology suite designed to scale Ethereum. You can use Arbitrum chains to do all things you do on Ethereum — use Web3 apps, deploy smart contracts, etc., but your transactions will be cheaper and faster. 

Why use Arbitrum? 1. Trustless security. 2. Compatibility with Ethereum. 3. Scalability. 4. Minimum cost.

Trustless security:  Security rooted in Ethereum, with any one party able to ensure correct Layer 2 result

Compatibility with Ethereum:  Able to run unmodified EVM contracts and unmodified Ethereum transaction

Scalability:  Moving contracts’ computation and storage off of the main Ethereum chain, allowing much higher throughput

Minimum cost:  Designed and engineered to minimize the L1 gas footprint of the system, minimizing per-transaction cost.

Why do I need ETH to use the Arbitrum network? When moving funds (ETH and non-ETH) from Ethereum (L1) to Arbitrum (L2), you'll need to have ETH in your wallet on the corresponding Arbitrum chain.