Most biological membranes will block larger molecules, like polysaccharides (carbohydrates). This is useful because it allows cells to hydrate themselves without absorbing useless waste. Osmosis is a passive process, but it is vital to survival. For example, plants use osmosis to absorb water through their roots. Water will flow from the soil into the plants roots, and then through the rest of the plant's cells that have been depleted of water. Because of the cell's selectively permeable membrane, all of the dirt and other wastes are left behind.
Roots absorbing resources
There are two major ways by which osmosis takes place, a solvent will either pass over the cell's phospholipid bilayer, or by passing through aquaporins, which are basically transmembrane proteins that create ion channels and aid in the process of diffusion.
One of the interesting properties of osmosis is something that most people have heard about, or seen for themselves. Most people have heard of the analogy of salting a slug, a very cruel thing to do. This is osmosis at work, the outside of the slugs body has a higher concentration of salt and a lower concentration of water, thus water will flow out of the slug and it will shrivel up and die. Fortunately, this same thing also works on leeches. Osmosis is also why you can't put saltwater fish into freshwater, or put freshwater fish into salt water. If you were to do this, your poor fish would die horribly, especially the saltwater fish. Before I finish, I'll just mention reverse osmosis. This is the process by which solvents can be filtered by passing them through a membrane, all the waste will stay on one side.
Without osmosis, life as we know it wouldn't be possible, in fact, I have a hard time imagining life at all without osmosis, though it's certainly possible.