Because the plasma membrane of cells selects what it wants to absorb and what it doesn't. It has carrier proteins within it, which only absorb certain ions or other molecules into the cell, whilst leaving others. A cell that isn't selectively permeable on the other hand absorbs everything it is able to when it comes in contact with it.
The plasma membrane is called semi permeable because it selects the things that it wants to let in .Hence it selects only useful items.
Because it allows only useful substances to enter it and does not allow unwanted substances to enter
Because it is for brunt heads