The waqf is the institution that explains how almost every mosque, madrasa, hospital and fountain of the pre-modern Muslim world was built and maintained without a state budget.
Inalienability is the whole thing
A waqf is a perpetual endowment: property given irrevocably so that its income supports a named purpose for as long as it lasts. The founder settles the asset, appoints an administrator and specifies the beneficiaries, and the asset can then never be sold, given away or inherited. That inalienability is the entire institution, and it made the endowment the ordinary mechanism of public provision, entirely outside the treasury. A second form, the family waqf, named the descendants of the founder as beneficiaries and functioned as an estate settlement placing property beyond both the fixed inheritance shares and the reach of confiscation.
What the modern state absorbed
By the nineteenth century a very large share of the cultivable land of Ottoman Egypt, Anatolia and Syria was held in endowment. The administrative reforms of that century and the states that followed absorbed the endowments into ministries almost everywhere. Whether that transfer destroyed an independent civil sphere or freed stagnant assets from dead hands is argued both ways, and the economic critique of Timur Kuran and the essays collected by Pascale Ghazaleh represent the two readings. What is not disputed is that the modern Muslim world does not possess the institution its predecessors did.