https://wccftech.com/steam-for-mac-is-now-64-bit-also-brings-new-design-and-chat-features/

Developers are gearing up for the final release of macOS 10.14. Ahead of the public release, Steam has updated its Mac app with support for 64-bit. Not only that the update also brings new design changes as well as n=forward-facing additions. The 64-bit support is important for Steam and other developers because macOS Mojave will be the last version which will support 32-bit applications. So let’s dive in to see some more details on the matter.

It was noted in a Reddit thread that update to 64-bit support gives Steam a gigantic performance boost. The interface is now more responsive and the overall user experience is “significantly better” compared to the 32-bit version of the app. However, performance is not the only aspect which was kept in mind when developing the update.