Where did the idea of the mihrab in mosques come from? And what is its function?

Initially, the term "mihrab" referred to a fortress from which an enemy was fought, and then to a fortified palace, as in the Almighty's saying: "They made for him what he willed of mihrabs and statues." It then came to refer to a house where one secluded oneself for worship, so Zechariah and David, peace be upon them, had mihrabs for worship, as in the Almighty's saying: "Then the angels called him while he was standing praying in the mihrab" and "When they climbed over the mihrab." As for the mihrab in its modern sense, it appeared later. It is said that the first one was placed in the Umayyad Mosque in Damascus during the reign of Al-Walid ibn Abd al-Malik, and its primary function was to indicate the qiblah. It then became a small niche where the imam stood.