The Ka'bah is the the first place of worship built by the direction of Allah himself. It was first built by Adam, then after the flood of Noah pummelled it it was built again by Abraham and Ishmael. Now it is the direction Muslims face during their prayers. It is important to note that though Muslims face the Ka'bah during prayers, they do not worship the Ka'bah. Muslims worship and bow to none but Allaah.
It is mentioned in Surah Baqarah:
"We see the turning of thy face (for guidance) to the heavens: now shall We turn thee to a Qiblah that shall please thee. Turn then thy face in the direction of the Sacred Mosque: wherever ye are, turn your faces in that direction." (Al-Qur'ân 2:144)
1. Islam believes in fostering unity
For instance, if Muslims want to offer Salaah (Prayer), it is possible that some may wish to face north, while some may wish to face south. In order to unite Muslims in their worship of the One True God, Muslims, wherever they may be, are asked to face in only one direction i.e. towards the Ka'abah. If some Muslims live towards the west of the Ka'abah they face the east. Similarly if they live towards the east of the Ka'abah they face the west.
2. Ka'abah is at the Centre of the World Map
The Muslims were the first people to draw the map of the world. They drew the map with the south facing upwards and north downwards. The Kaaba was at the centre. Later, western cartographers drew the map upside down with the north facing upwards and south downwards. Yet, Alhamdullilah the Ka'abah is at the centre of the world map.
3. Tawaf around Ka'abah for indicating one God
When the Muslims go to Masjid-e-Haram in Makkah, they perform tawaf or circumambulation round the Ka'abah. This act symbolizes the belief and worship of One God, since, just as every circle has one centre, so also there is only one Allah (swt) worthy of worship.
4. Hadith of Umar (may Allaah be pleased with him)
Regarding the black stone, hajr-e-aswad, there is a Hadeeth (tradition), attributed to the illustrious companion of the Prophet Muhammed (pbuh), Umar (may Allah be pleased with him).
According to Sahih Bukhari, Volume 2, book of Hajj, chapter 56, H.No. 675. Umar (may Allah be pleased with him) said, "I know that you are a stone and can neither benefit nor harm. Had I not seen the Prophet (pbuh) touching (and kissing) you, I would never have touched (and kissed) you".
5. People stood on Ka'abah and gave the adhaan
At the time of the Prophet, people even stood on the Ka'abah and gave the ‘adhaan’ or the call to prayer. One may ask those who allege that Muslims worship the Ka'abah; which idol worshipper stands on the idol he worships?