[NotMyWay]

One use of material composition is to aid in leveling material decomposition by recycling materials.

For instance, after decomposing several copper ores into low quality ones, u could use material composition to compose the low quality back into normal quality and decompose again. Of course u will lose money in the process, but it is a faster way to raise ur decomposition skill since u wont need so many materials. Also, some materials may have better exchange rates than others, which u might want to consider before using this method.

for example:
4 copper ore (LQ) >> 1 copper ore
1 copper ore >> 2 copper ore (LQ)

whereas
11 spoiled mushroom (or was it 15?) >> 1 common mushroom
1 common mushroom >> 9 spoiled mushroom
(much better exchange rate)