Why it is necessary to stand towards Kiblah (the Holy Ka’bah) while praying, while Allah is everywhere and does not have a specific direction?
We have to face Kaaba for unity of Muslims. This is true Allah is everywhere, but It Important to unity humans. But there is also a History of it.
How it was built by Abraham and Ishmael.
Detail in Wikipedia
The Quran states that Ibrahim, together with his son Ishmael, raised the foundations of a house [Quran 2:127] that is identified by most commentators as the Kaaba. Allah had shown Ibrahim the exact site, very near to the Well of Zamzam, where Ibrahim and Ishmael began work on the Kaaba's construction in circa 2130 BC. After Ibrahim had built the Kaaba, an angel brought to him the Black Stone, a celestial stone that, according to tradition, had fallen from Heaven on the nearby hill Abu Qubays. According to a saying attributed to Muhammad, the Black Stone had "descended from Paradise whiter than milk but the sins of the sons of Adam had made it black". The Black Stone is believed to be the only remnant of the original structure made by Ibrahim.
After the placing of the Black Stone in the Eastern corner of the Kaaba, Ibrahim received a revelation, in which Allah told the aged prophet that he should now go and proclaim the pilgrimage to mankind, so that men may come both from Arabia and from lands far away, on camel and on foot.[Quran 22:27] Going by the dates attributed to the patriarchs, Ishmael is believed to have been born around 2150 BC, with Isaac being born a hundred years later.
Therefore, Islamic scholars have generally assumed that the Kaaba was constructed by Ibrahim around 2130 BC. The Kaaba is, therefore, believed by Muslims to be more than a millennium older than Solomon's Temple in Jerusalem, which is believed to have been finished in 1007 BC. These dates remain consistent with the Muslim belief that the Kaaba is the first and thus oldest mosque in history.
In Samaritan literature, the Samaritan Book of the Secrets of Moses (Asatir) claims that Ishmael and his eldest son Nebaioth built the Kaaba as well as the city of Mecca. "The Secrets of Moses" or Asatir book was suggested by some opinion to have been compiled in the 10th century, while another opinion in 1927 suggested that it was written no later than the second half of the 3rd century BC.
As it is an important place because it was built for Allah. But when Islam came, Kaaba was not where Muslim face to pray. At first Muslim use to face towards baitul muqaddas then it was change because Muhammad (PBUH) ordered it. When Muhammad (PBUH) order something it has to be done in that exist way, no other way will be accepted.
According to the holy Qur’an, (Surah al-Baqarah: 115):
وَلِلَّـهِ الْمَشْرِقُ وَالْمَغْرِبُ ۚ فَأَيْنَمَا تُوَلُّوا فَثَمَّ وَجْهُ اللَّـهِ ۚ إِنَّ اللَّـهَ وَاسِعٌ عَلِيمٌ
And to Allah belongs the east and the west. So wherever you [might] turn, there is the Face of Allah. Indeed, Allah is all-Encompassing and Knowing.
But with respect to the holy Qur'an (and likewise narrations) according to other verse(s), we ought to do some deeds toward the Kiblah.
قَدْ نَرَىٰ تَقَلُّبَ وَجْهِكَ فِى ٱلسَّمَآءِ ۖ فَلَنُوَلِّيَنَّكَ قِبْلَةًۭ تَرْضَىٰهَا ۚ فَوَلِّ وَجْهَكَ شَطْرَ ٱلْمَسْجِدِ ٱلْحَرَامِ ۚ وَحَيْثُ مَا كُنتُمْ فَوَلُّوا۟ وُجُوهَكُمْ شَطْرَهُۥ ۗ وَإِنَّ ٱلَّذِينَ أُوتُوا۟ ٱلْكِتَـٰبَ لَيَعْلَمُونَ أَنَّهُ ٱلْحَقُّ مِن رَّبِّهِمْ ۗ وَمَا ٱللَّهُ بِغَـٰفِلٍ عَمَّا يَعْمَلُونَ
We have certainly seen the turning of your face, [O Muhammad], toward the heaven, and We will surely turn you to a qiblah with which you will be pleased. So turn your face toward al-Masjid al-Haram. And wherever you [believers] are, turn your faces toward it [in prayer]. Indeed, those who have been given the Scripture well know that it is the truth from their Lord. And Allah is not unaware of what they do. (Surah Al-Baqarah: 144)
As a result, in accordance with the verse above, we ought to be toward qiblah in our Salah.
On the other hand it has shown in the traditions that we must say prayers toward kiblah.
حر عاملي، وسائل الشيعه، نشر دار الاحياء التراث العربي، بيروت 1412 ق. ج4، ص 295. (Hor Ameli, Wasael al Shia)
We can pay attention to the mentioned issue according to the issue of Unity. In truth, Muslims must pay attention that there must be an aim behind the issue. In fact, there must be a goal if Allah order us to prayer toward a specific place. Accordingly it could be rational that the significant point of the praying toward the Kaaba is related to the unity of the Muslims. Since as you are aware, as a rule, have the unity will be the cause of the power between different Muslims of the world (in different country).
So, I assume you inquired a very significant issue, since it could remind us not to forget the significance of the unity (Wahdah) between Muslims.