وَصَلَ الْعُمَّالُ وَهُمْ يَحْمِلُونَ الْأَدَوَاتِ.
The workers arrived while carrying the tools.
Hum is the subject of a nominal clause whose predicate is the verbal sentence yaḥmilūna al-adawāt. The entire nominal sentence describes a circumstance; the preceding wāw is circumstantial wāw. The pronoun refers to the workers, who are the referent of the circumstance.
وَصَلَ الْعُمَّالُ وَحَمَلُوا الْأَدَوَاتِ.
The workers arrived and carried the tools.
If ḥamalū is presented as a separate action after arrival, the wāw coordinates actions instead. The decisive distinction is between a simultaneous accompanying condition and another independently predicated event.