Is there a case where using type witness is absolutely needed?

Prior to Java 8 yes. There would be the error for calling snippet code:

void processStringList(List<String> stringList) {
    // process stringList
}

And then if you call this method with collection.emptyList() compiler could not infer the type.

processStringList(Collections.emptyList());

The error would be List<Object> cannot be converted to List<String>

So you had to write it this way:

processStringList(Collections.<String>emptyList());

In JDK 8 and after compiler can infer it base On the method definition that the type in String. Then this will work:

processStringList(Collections.emptyList());

For more information check the Type Inference