We identify sugar with sweetness, which is one of the four main tastes, along with bitter, sour and salty (and umami). These tastes correspond to particular taste buds on the human tongue. A taste bud activates when a molecule of the appropriate shape touches it, then sending a message to the brain indicating what type of taste it's dealing with.
Besides evoking pleasure, sweetness can indicate that a food is safe to eat. Many naturalists advise nibbling on a tiny portion of a wild plant to determine if it's poisonous or not. A bitter taste indicates that the plant is likely unsafe. (Note: Don't get any ideas. Eating unfamiliar plants is still strongly discouraged.)
The sweet taste of sugar helps plants to attract bees, insects and animals that aid in pollination. Sugar also provides essential energy for plants.
In the human body, glucose is used for cellular respiration. For this reason, it is often referred to as "blood sugar" because it travels through the blood and diffuses across cell membranes. But to get glucose (and fructose), the body must break down sucrose, a task aided by the enzyme sucrase.
While sugar essentially powers human cells, it's still basically empty calories. It provides a quick boost of easily processed energy but little else. Sugary foods rarely have any of the other carbohydrates, proteins, vitamins and minerals necessary for a healthy diet. And as you'll recall, any excess sugar in the body won't be used as energy and will instead be stored as fat.
Obesity, which excessive sugar consumption certainly can contribute to, boosts the chances of developing type 2 diabetes. Also, high blood-sugar levels cause the body to produce an excess of insulin, potentially damaging the pancreas. It can hinder the passage of blood proteins. There are various types of diabetes, and the complications from the disease are potentially fatal -- another reason why sugar should be consumed in small to moderate quantities.
Finally, mind your teeth: Sugar is a leading cause of cavities. Glycoproteins from sugar stick to the teeth and become magnets for bacteria. The bacteria eat the fructose in sugar and produce lactic acid as a byproduct. The lactic acid can contribute to the wearing down of tooth enamel and the formation of cavities.