The cell membrane is selectively permeable, that is, it allows the entry to only a few molecules, while barring all the others. If the cell membrane was completely permeable, all the molecules will gain access to the cell interior. These molecules may include toxins and will be able to harm the cell or kill it. Thus, the semi-permeable or selectively permeable nature of cell membrane is necessary for cell survival and routine operation.
The transport of molecules across the membrane is necessary, since the cell cannot produce all the molecules it needs for routine operation and also to dispose off (or excrete) the molecules that are not useful to the cell. Thus, selectivity in terms of transport of molecules (between the cell interior and exterior environment) is necessary.
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