Dear student,
If we do not use weedicides on weeds than the weeds will grow more in size and occupy a lot of space which will interfere with the growth of our desired main crop.Apart from space, the weeds will also consume majority of nutrients from the soil and will lead to its depletion in the soil.The faster consumption of these essential nutrients by weeds from the soil will ultimately affect the health, growth and development of our plant and it will not grow properly.
The weeds also occupy the sunlight by growing over the main plant, thus the main plant will not get proper sunlight and will not be able to perform photosynthesis and it will die.
Regards