Why do cells need a selectively permeable barrier?

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

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.