All the plants which we eat are made up of sugars joined up very very tightly in the form of cellulose.
When we eat vegetables, (plants too) we are not very good at breaking this cellulose down, so this helps flush the rest of our foods along our digestive system which is a good thing because it gets rid of what we don’t need. We get our sugar much more readily from other foods, so we only need one stomach
Cows on the other need to break this down, since they only eat grass. As i said earlier this cellulose is difficult to break down and get the sugars, so the cow has to take its time and break this down in stages.