Among the problems encountered by farmers in their crops are weeds and their cousins -- grasses, vines, unwanted herbs, other short plants. And among the "-cides" that farmers resort to, is buying herbicides to control said weeds. Oh, the other "-cides" are pesticides, insecticides, fungicides, etc. to control other problems in the crops.
But crops, from rice and maize to vegetables and bananas, deplete lots of minerals in the soil as they mature and produce grains, edible leaves and fruits. So that in many agricultural literatures, the application of fertilizers, say a certain mixture of N-P-K, is a must if farmers want to continue getting high yields.
In our experience in our farm, those weeds can be our allies. But we have to put lots of time cutting them, if not uprooting them, then put them above the roots of trees and plants as mulch, and wait for them to decompose naturally. Of course the more weeds you put there, the higher the pile becomes, and new soil from decomposed organic matter is being created. Then we need to put stones around these piles so that they won't be eroded by raintwater or strong winds. But overall, after these laborious activities, we have lots of new rich soil, lots of organic fertilizers to replace the depleted minerals, and better yield from the crops and fruit trees.
If those weeds are not allowed to grow because of herbicides, then there will be more evaporation from the soil since those grasses and weeds help cover the soil. There will also be more topsoil erosion during heavy rains since those grasses and weeds help absorb strong raindrops before they reach the soil. And there will be less materials for production of organic fertilizers. So those weeds can be a farmer's ally somehow, huh? Yes, but the farmer must put extra effort and labor to manually cut and uproot them when they become big and tall and use them for mulching.
But what if the area is wide? It will be almost impossible for the farmer/s to manually cut and uproot those weeds. One solution are those ruminants like cows and goats. They feed on the grasses, make the area relatively clean, while leaving animal wastes that can help fertilize the soil. And when those animals are mature enough, they can be sold for cash or slaughtered for food.