A: The following Hadith explains the reason for it. Al-Saib ibn Yazid says: “The call for Friday prayers was first as the imam sat on the pulpit. This was the case during the time of the Prophet (peace be upon him), Abu Bakr and Umar. When Uthman was the Caliph and people increased in number, he added the other call made at Al-Zawra.” (Related by Al-Bukhari). When the Prophet settled in Madinah and built his mosque, his companions came over to attend the regular prayers without there being any form of announcement. They simply came when they felt the time approaching. They waited until it was time. However, the Prophet and his companions felt that this was inconvenient, because people could be busy with their work and do not realize that prayers were due. The Prophet discussed the matter with his companions, and eventually the adhan was established as the form of announcement that prayer was due.
At the time of the Prophet, the adhan for Friday prayer was made inside the mosque, when he sat on the pulpit, ready to give his sermon. This continued to be the case during the reigns of Abu Bakr and Umar. However, the numbers of Muslims in Madinah continued to increase. Uthman felt that as people went about their business on Friday, they needed to be reminded of the time for prayer a short while before it started. Hence, he ordered that a fresh adhan be called outside the mosque, at Al-Zawra. What he added then is the new form of a call made a few minutes before the prayer. He did not add anything that was not part of the prayer. His reason for adding it was to make it easier for people to realize that the time of the prayer was due and to get ready for it. All the Prophet's companions living in Madinah at the time agreed that his action was sound and useful.
Al-Zawra, where the adhan was made was an elevated spot at the market place. Some people also said that it was a large stone close to the door of the mosque. Whichever was the case, it was an elevated spot to make the call better heard. When people gathered inside the mosque and the imam sat on the pulpit, a new adhan was made, to conform to the practice that was done during the Prophet's lifetime. Thus nothing of that practice was changed. The only addition was the new call, a short while before the prayer, to enable people to get ready.
Nowadays, the practice established by Uthman is followed throughout the Muslim world, and rightly so. Without it, many people are likely to miss out on Friday prayer, particularly because life in modern cities makes it difficult for people to be always mindful of the prayer times. All other prayers have an extended time range, so people can adjust their time easily to pray on time. Friday prayer does not have a similar range, because of its particular nature and the requirement that it should be prayed in the mosque, preceded by a sermon. The addition by Uthman ensures that people are reminded in good time and can easily put down whatever they are engaged in to go to the mosque and attend the prayer.