Thus they share electrons with their neighbors, in such a way that including the shared electrons the shells are full. These are stronger bonds than either of the other two types. This is because the electrons are shared. The carbon bond usually forms covalent bonds. The possible arrangements are many.
Life is based upon carbon-carbon bonding.
The branch of science called organic chemistry is the study of carbon-based molecules. Your body is based upon carbon bonding. So the covalent bond is considered the most important bond with regards to life. Interestingly, Si, just above C in the periodic table, with its covalent bonding, is the basis for the computer industry. Another way to look at these things.