Many modern Jews think that the laws of kashrut are simply primitive health regulations that have become obsolete with modern methods of food preparation. There is no question that some of the dietary laws have some beneficial health effects. For example, the laws regarding kosher slaughter are so sanitary that kosher butchers and slaughterhouses have been exempted from many USDA regulations.
However, health is not the only reason for Jewish dietary laws. Many of the laws of kashrut have no known connection with health. To the best of our modern scientific knowledge, there is no reason why camel or rabbit meat (both treif) is any less healthy than cow or goat meat. In addition, some of the health benefits to be derived from kashrut were not made obsolete by the refrigerator. For example, there is some evidence that eating meat and dairy together interferes with digestion, and no modern food preparation technique reproduces the health benefit of the kosher law of eating them separately.
In recent years, several secular sources that have seriously looked into this matter have acknowledged that health does not explain these prohibitions. Some have suggested that the prohibitions are instead derived from environmental considerations. For example, a camel (which is not kosher) is more useful as a beast of burden than as a source of food. In the Middle Eastern climate, the pig consumes a quantity of food that is disproportional to its value as a food source. But again, these are not reasons that come from Jewish tradition.
The short answer to why Jews observe these laws is: because the Torah says so. The Torah does not specify any reason for these laws, and for a Torah-observant, traditional Jew, there is no need for any other reason. Some have suggested that the laws of kashrut fall into the category of "chukkim," laws for which there is no reason. We show our obedience to G-d by following these laws even though we do not know the reason. Others, however, have tried to ascertain G-d's reason for imposing these laws.
In his book "To Be a Jew" (an excellent resource on traditional Judaism), Rabbi Hayim Halevy Donin suggests that the dietary laws are designed as a call to holiness. The ability to distinguish between right and wrong, good and evil, pure and defiled, the sacred and the profane, is very important in Judaism. Imposing rules on what you can and cannot eat ingrains that kind of self control, requiring us to learn to control even our most basic, primal instincts.
Donin also points out that the laws of kashrut elevate the simple act of eating into a religious ritual. The Jewish dinner table is often compared to the Temple altar in rabbinic literature. A Jew who observes the laws of kashrut cannot eat a meal without being reminded of the fact that he is a Jew.
