Answer:
The cell membrane of the cell has multiple uses.
Firstly, it acts to compartmentalise cellular processes from the extracellular environment. This helps to maintain a steady and constant internal cell environment for processes to take place.
Secondly, it acts as a membrane barrier for entry of substances into the cell. The cell membrane is partially permeable and only allows certain particles to pass through its phospholipid bilayer, namely small hydrophohic molecules such as glucocorticoids. Entry of other substances would require protein transporters to transport proteins into the cell.
Thirdly, the cell membrane allows for the cellular signalling and cell-cell interaction with other cells in the organisms. There are numerous receptors on the cell's surface and embedded in the cell membrane which allows for cell communication. For example, if it is infected by pathogen, pathogenic antigens would be expressed on the cell membrane and immune cells would respond accordingly.