These cells synthesize lytic substances that work intracellularly on phagocytized micro-organisms, cellular debris, excretions or anything else the body does not want or need. The more surface area on a cell, the more nutrients the cell may take up and the more waste that may be efficiently excreted. The more a cell takes up and utilizes, the larger the cell. This membrane has three-dimensional structure, although we'll examine it from only 2 dimensions. Note that it is a lipid bilayer membrane, i.e., there are two layers of lipid that surround the cell.
Water is a polar molecule.
The value of this is that the outer layer is a bit more rigid and the inner layer is more flexible, much like taking two sheet of corrugated aluminum roofing and layering them, then bending them. With the aluminum roofing, the inner layer seems to extend beyond the edges of the outer layer when it's bent. By making the inner layer of a membrane more flexible, it bends, as it were, to retain its alignment with the outer layer without extending beyond the outer layer. Cholesterol, as mentioned before, works with the rigidity of the membrane. The latter proteins are often-times ion channels, as their outer layer is hydrophobic and more interactive with the cell membrane, while the center portion is hydrophilic.