All mirror cells share the basic functions of supporting the Main Mirror in the Telescope Tube in such a way that the Main Mirror can be tilted in any direction and locked in any new position thus reached, - so that the telescope can be collimated. The Main Mirror is quite massive, so its supporting cell must be able to keep it aligned when the telescope is swung around to point at a new object in the sky. The cell must absorb the mirror's momentum when the motion stops.  Otherwise the mirror will be jerked out of alignment with the rest of the telescope's optics.
For any specified mirror diameter, the shorter the focal length, the steeper the curve that is ground into its surface.  If you used the mirror as a bowl to hold water, the depth of the water in the center of this shallow bowl would be at its deepest. The depth of the water at this point is the mirrors Sagitta.  Of course we don't use the mirror as a bowl, but the measurement of the mirror's sagitta is an important property of the mirror.
It is readily apparent that steeper curves have larger sagittas. The sagitta of a dinner plate is very large compared to the sagitta of a telescope's main mirror. However, no dinner plate ever had such a finely honed reflecting surface where the margin of error in the curve must be smaller than 0.0000001375 meters before the instrument even qualifies as a telescope.  The problem is that glass is not a solid, it is a highly viscose liquid that flows.  If the mirror's momentum is not absorbed by its support, some of the glass in the hills of the still not perfect surface, will flow into some of the valleys of the surface, disfiguring the mirror at a microscopic level. At the same time, the mirror's overhang will sag, altering the mirrors fine figure at the macroscopic level as well.