Ryu - a lot of wattles start of by having leaves, which they shed after the first few years in favour of flattened stems, called phyllodes. The Leaves may help the wattle grow quickly and get above the competition for light from under storey grasses and other plants, while shedding the leaves once above the competition probably helps to conserve water as there are less stomata, or breathing holes ,for water to transpire out of in the stems