Ok, I'm not 100% sure this is correct, given the context of the situation, but basically, the cell membrane helps the cell maintain homeostasis by keeping what needs to be in in & what needs to be out out. If it weren't for the cell membrane having the structure it does, all kinds of things could get into the cell & completely kill it from filling it with too much stuff, or the wrong things could get in (say, things with a really odd pH) & damage it badly. Or, it could let things get out extremely easily, such as enzymes and ions the cell needs to function properly.
Hope this helps!
