How does the cell membrane help maintain homeostasis?
It allows material to stay in and prevent outside material from entering
As you know, homeostasis is the ability to maintain living conditions. Cells need certain conditions just like you do. What's more, cells are susceptible to whatever they are in. To stop osmotic pressure changes, they have a membrane. To stop particles they need from leaving their "bodies" they have the membrane. To only allow very little particles to enter unless absorbed with a vesicle they have the membrane. They also use the membrane in chemical processes and diffusion of material, like pumping protons or making ATP. All these processes they need, and the materials as well.
TL:DR It's a biochemical scaffold, a semipermeable fluid barrier, resists some pressure, and allows import and export of material. All things the cell needs to maintain life.