It is very important for the cell membrane to be <em>semi-permeable</em> because the ability to pick and choose what comes in and out of the cell keeps the cell safe! The membrane can choose to block out nasty germs and can also choose to get rid of internal waste.
An example of when this is important is in this scenario:
Let's say the cells are in a really salty solution. Naturally, salt will want to pull the water out of the cell. If the membrane <u>wasn't</u><u> </u> semi permeable, the water would listen to the salt and leave the cell. This would cause cell death. <u>BECAU</u><u>SE</u> the membrane is semi permeable, they can choose <em>not</em> to give the salt any water, keeping them alive
To reproduce (asexually). Most unicellular organisms only live for mere hours. The best bet for the species to survive and thrive as a whole is for them to reproduce, or replicate.
