Because Baghdad was the capital, there was a constant flow of scholars seeking and bringing knowledge, and traders bringing in books from all over. Some of the greatest scholars and philosophers in the Islamic world came from Baghdad, including al-Kindi.
Under al-Rashid, the House of Wisdom began a translation movement, gathering manuscripts in Chinese, Greek, Persian, Sanskrit, and Syriac to turn them into Arabic. Astronomy, mathematics, medicine, and philosophy were important, but so was poetry. Under his son, al-Ma'Mun, the House of Wisdom expanded due to increased financial support, and translators not only made a good living but were considered to be of high social status. al-Ma'Mun was said to appreciate science more than the spoils of war. There is a story that al-Ma'Mun had a dream in which he and Aristotle discussed what is good. The caliph would regularly visit the House and engage in philosophical debates.
Not just a center for learning, the House also trained architects, engineers, medics, and civil servants. al-Ma'Mun organized scholars to map the world, to accurately determine the size of the world, and he was personally involved in excavations of the pyramids at Giza. He built the first astronomical observatories in Baghdad and funded major research projects. Al-Ma'Mun was the first ruler to fund what is sometimes called "big science."
Long before there were formal universities, the House of Wisdom was a place where all learning was encouraged and taught. Tomorrow, sadly, we will see how it was all destroyed.