Question

How does a selective permeability affect a cell?

Answer 1

In many ways (see in explanation)

Explanation:

1st way: It controls what exits and enters the cell in response to its needs and to avoid entry of harming for the cell molecules and substances.

2nd: It allows the cell to have specific mechanisms to receive special substances (diffusion— and more specialized processes like facilitated diffusion, passive transport and active transport).

3rd: Depending on temperature it can adjust its permeability to in a way insulate the interior of the cell and protect the organelles by preventing high or low temperature substances enter.

4th: It is pertinent to cellular communication using the selective permeability to allow acceptance and entry of other cells' "information molecules" or neurotransmitters (for nerve cells only).

and
5th way: It effectively protects the cell from physical harm due to its structure (see fluid mosaic model)

These are the ones that come to my mind. If you need any more info please ask :)