Generally gets attributed with:

- Increased liquidity. Ensures there's actually something to be traded available globally, and swiftly moves it to places where it's lacking.

- Tighter spreads, the difference between you buying and then selling again is lower. Which often is good for the "actual users" of the market.

- Global prices / less geographical differences in prices. Generally you can trust you get the right price no matter what venue you trade at, as any arbitrage opportunity has likely already been executed on.

- etc..