You’re right, chemistry was not your strong point. The sugars produced by photosynthesis are the ones that get turned into coal and oil. Plant respiration actually is the reverse of photosynthesis (it’s essentially human respiration) and is done for exactly the same reason that humans use it for, to produce energy for use by the plant. A plant just does more photosynthesis than respiration, which causes it to grow over time as it accumulates carbon based molecules and stores them.