From their website:
Immutable data cannot be changed once created, leading to much simpler application development, no defensive copying, and enabling advanced memoization and change detection techniques with simple logic.
You can see immutable being used a lot in combination with Redux, because it's important for reducers to be pure. It allows for better testing and for functions that don't depend on specific input. This prevents a lot of bugs from happening, ultimately leading to a better quality of code.