Why is possible to write a function outside a class in Kotlin?

Yes, this is good practice. Kotlin is not a purely object-oriented language, so it's not obligated to follow how "an object-oriented language works normally" (even though other object-oriented languages, such as C++, Ruby and Python, also allow top-level functions).

It's better to use a top-level function when the logic of this function does not clearly belong to any class.

In Java, this is impossible! That's not how an object-oriented language works normally, right?

Just stop for a second and reconsider the nature of java's static method. A class is supposed to be a blueprint for objects, describe their behavior and state. But you can call a static method without creating any instances.

How does that fit into the object-oriented picture? How does a static method "belong" to the class it's declared in?

Actually static methods are a hack in Java, they pollute and misuse the OOP notion of a class. But you got used to them over the years so you don't feel that anymore.

Conceptually, a static method is a top-level function and Java uses the name of its declaring class as its namespace. In contrast to that, Kotlin allows you to declare top-level functions without misusing the class for namespacing.