Sometimes what is important is not what atoms are bonded covalently, but how molecules are held together loosely. Intermolecular forces are often unappreciated, however, they are the glue that holds our molecular machinery together. Not to get too biology on us, but proteins, DNA, and cell membranes are all held together by intermolecular forces.
Intermolecular forces work well for biological things. They are not overly strong, so cell processes can yank them apart and put them back together with relative ease. For example, DNA double helices must be pulled apart for cells to reproduce. If the DNA double helix was held together strongly with covalent bonds, then cells would have to work extra hard to reproduce. Instead, DNA is held together with hydrogen bonds, which are much weaker than covalent bonds. This makes it easier to unzip DNA, and then rezip it.
