The point is similar to the idea of clearing a pointer in a C-language (if you’re familiar with that). It is used to make the data associated with a ready for garbage collection so memory/data is not inefficiently bound to a when it doesn’t need to be. Read more here.
It’s important to note that this isn’t really necessary unless you’re actually reusing “a”. GC should automatically clear up this data for you if the object is not used again. The reason why a TypedArray is different, however, is because a TypedArray has other internal data that must be returned (known as StyledAttributes) to the TypedArray for later reuse. Read about that here.