The Mekhilta concludes its treatment of the Sabbath fire prohibition with a clean summary of the legal principle. Lighting a fire was one of the thirty-nine proto-labors forbidden on the Sabbath. It was part of the general class. But the Torah removed it from the general class and mentioned it individually.
Why? To teach by example. Just as a person is liable for lighting a fire — a single labor out of thirty-nine — in and of itself, so too is he liable for each of the other thirty-nine proto-labors in and of itself. Each labor is an independent prohibition carrying its own independent penalty.
This is the principle of "singled out from the general class to teach about the entire class." When the Torah extracts one item from a list and gives it special treatment, the special treatment is not limited to that one item. It applies to every item in the list. Fire was chosen not because it is different from the other labors but because it is representative of them. Its individual mention establishes a rule that governs all thirty-nine categories.
The practical impact is that a person who violates multiple Sabbath labors in a single day is liable for each one separately. Writing and cooking and carrying on the same Sabbath means three violations, three penalties. The labors do not merge. Each one stands alone — just as fire stands alone in its verse, teaching that independence applies across the board.