There is a saying..crops are sown by man while weeds are sown by god. If your crop is surrounded by weeds and starts blocking sunlight, the crop growth will be arrested. A matured weed plant produces hundreds of seeds. Once produced these seeds may get carried away by wind, rain water or may germinate in the same place under right conditions. Obviously to reduce the weed seed population, it is best to cut them before seeds set in. This way the weed population can be controlled effectively. Other sources of weed seeds are tilling, wind and rain water. If we avoid tilling, and mulch heavily, then seeds which typically comes with wind will remain mostly on surface and won't germinate. Also the rain water from other areas should not be allowed to flow to your field which will have many weed seeds.
Also it is better to time the weed cutting and mulching well advance before the rain recedes so that moisture can be retained. For e.g in Kerala, experts advice to mulch before the north-east monsoon so that the rain water from this monsoon can be well preserved.
