I am not absolutely sure on this, but I belive it is to emphasize in a taxonomical description, the name of the organism. While genus and species are also taxonomical groupings, same as family, order, etc., the actual binomial name of an organism consists of a genus and species. For example, Acropora millepora is the name, not Acroporidae Acropora millepora. btw, genus is italicized - or should be - even when talking about a group.
Here is a little bit on the subject:
"Names of organisms. Biological organisms have an official scientific name (called a latin binomial), as well as one (to several) common names. Common names vary regionally, so clear communication requires the used of scientific names. Scientific names come in two parts (thus, binomial), the first part the genus (plural, genera), the second part the species (plural, species). The genus names is always capitalized, and the species name is never capitalized (even if it is a proper noun!). The genus and species names are always italicized or underlined. Recent changes in the nomenclatural code also requires family names to be italicized, but this practice is not universally adopted (they should always be capitalized, however). Following the species name, one sometimes finds additional words. They may be a designation for a subspecies or variety (designated by a non-italicized "ssp." or "var." followed by the italicized name), or may designate the "authority", the person who gave the organism its name. The authority is never italicized, and should be given with the organism's name the first time it is used in a paper, but not thereafter. The first time that the organism is named in a paper the full genus and species must be written out; after that, the first letter of the genus name, followed by the full species name may be used. Common names should be capitalized, but never italicized. "