Why do cells need a selectively permeable barrier?

Using a selectively permeable membrane, a cell can make sure that no harmful materials enter the cell while still obtaining nutrients from its environment. If a cell did not have a selectively permeable membrane, malignant materials could enter the cell and damage or destroy it or even make it cancerous. If it didn't have a selectively permeable membrane it would also have no way of separating its own components from the environmental components around it.

Well so only good stuff goes in and only bad stuff comes out.