Urns are a common prop used on stage in Greek Theatre both in ancient times and today. This reflects their use in the daily life of the Greeks as storage for both everyday items, as well as more precious items such as expensive perfume.
Brass urns were rare so they would have been for a special, ceremonial function of some kind. An important use was funereal to hold the ashes of the dead. Urns known as "lekythos" were preferred -- these urns have a long, slender cylindrical shape with a narrow neck.
When props are used in Greek Theatre, they usually also have a deeper significance and signaled deeper themes. The Greek stage consisted of a circular orchestra or "dancing space" where the chorus performed and a rectangular stage behind it where the actors performed. With audiences of more than 14,000 people, the staging was limited. The audience would have paid close attention to anything appearing and a playwright would have intended every prop to be symbolic.
Sophocles' play "Elektra" features an urn in a starring role. Elektra longs for revenge against her mother who killed her father, King Agamemnon of Mycenae, when he returned from the Trojan War. She also grieves for her brother, Orestes, whom she helped escape when their mother also attempted to kill him. Orestes hatches a plot to return for his revenge, announcing his false death and arriving with an urn delivering his own supposed ashes to Elektra. When Elektra takes the urn, she laments for his death far away and Orestes is moved to reveal himself and the empty urn. It was recorded famously that when the ancient Greek actor Polus played the part of Elektra, he placed the ashes of his own deceased son in the urn.