Because it is closer to "Taková slova jsou pro nás zajímavá." We must require the translations to be as close as possible else the number of the possibilities would be immense.
However in this case even the official answer is not an exact one to one translation. You do have some point here.
It has the same meaning and is used to add stress. It is somewhat more strongly pointing to the object. Etymologically the -hle comes from the imperative hleď=see.
It is part of the spoken form of standard Czech and is common in coloquial speech. Some even think it is nonstandard (see other comments).