If you use iReal Pro on multiple iCloud enabled devices, such as a Mac, an iPad and an iPhone, iCloud Sync allows you to keep your playlists and songs in sync between all of them.

When you make changes on one device, such as adding a new song, a new playlist, transposing a song, etc., these changes will automatically be saved to iCloud.
The next time you open iReal Pro on another device it will automatically download these changes and update the songs and playlists.

If you only use iReal Pro on one device, you can still use iCloud Sync to keep a copy of all your songs and playlist safely stored in iCloud in case your device breaks, is lost or when you decide to upgrade to a new one.