What is the significance of the -R group (variable radical) in an amino acid molecule?

The -R group, also known as a lateral chain, is the variable part of the amino acid molecule. The -R group can be a complex carbonic chain, a substituting methyl group (forming then the amino acid alanine) or even only a hydrogen (forming glycine, the simplest amino acid). So the -R group is significant because it is the differentiation factor of amino acids.