What is the structural function of a pendentive in supporting a dome, as seen in Turkish multi-domed Mosques?

The pendentive allows the support of the circular base of a dome over a square or polygonal space. In a nutshell, it avoids the dome to fall out.

Explanation:

The pendentives, which are triangular segments of a sphere, taper into points at the bottom and spread at the top to establish the continuous circular or elliptical base necessary for a dome. The pendentives were mainly used in Renaissance and Baroque architecture. The first experiment with the use of pendants was carried out in Roman domed buildings in the early 2nd and 3rd centuries.