What's the significance of second degree skin burns?

Matter of degree. There are 3 degrees of burn-1, 2, and 3 graded by severity. Sunburn would be a typical 1st degree burn, blistering from steam or a hot surface that is very painful would be typical for grade2. In grade3 burns the injury is so severe that the pain nerves are destroyed and paradoxically there is less pain. The crucial factor is how much surface area of the skin is involved.
Needs treatment. The significance is that the skin has been significantly damaged, and needs treatment. Urgency depends on the number and extent of the burns, and whether they are located near vital structures, such as the eyes.