Separation of pure and unpure components greatly simplifies the reasoning about the problem domain. Traditional imperative programs are often full of subtle bugs because calling a procedure can have arbitrary effect on your system. For example calling function with the same arguments can return different values based on arbitrary hard-to-track reasons (such as your OS's scheduler). It's just too much details to keep in your head.

But in Haskell pure functions are guaranteed to have same result with same input. It enables the programmer to create logically isolated blocks without messy interdependencies.

It's especially helpful for concurrent programming by freeing you from all the non-deterministic spaghetti.

Haskell has a steep learning curve, but it'll make you a better programmer in the long run.