Farmers used to plough fields after harvest. This kills weeds. It also uses a lot of power, so burns a lot of diesel and releases CO2. Ploughing also allows carbon trapped in organic material in the soil to break down and releases yet more CO2 as well as destroying soil structure.
Low/no till farming seeds directly into last year's stubble. This uses much less diesel so releases much less CO2. Organic matter in the soil builds up, trapping more carbon, and improving soil health. But now the weeds grow unchecked and destroy the crop.
A common variation of the above low till method it to 'cultivate' (sort of scuff up) the surface of the soil after harvest. This causes weeds to germinate. Then you spray them with glyphosate (Roundup), and can almost immediately start sowing your crop. This allows you to farm with much less carbon released, lower fertilizer usage (also reducing carbon).
Roundup allows farming with lower inputs and better soil health and less carbon released. That is why so many people defend its use.