Why do they have to move cattle?

Cattle need to be moved from one pasture or grazing area to another because they, just like any herbivore, tend to eat/trample the grasses down where they have grazed, limiting their ability to get more to eat for themselves. Cattle tend to consume a lot in a short period of time (if you leave them in a small paddock for a little while), and thus need to be moved before they overgraze that particular area. The amount of feces in that particular paddock can also determine whether you need to move your cattle to a fresh paddock or not.

Cattle are more likely to overgraze an area (but not nearly as bad as sheep and horses are) if they are not moved soon enough. Overgrazing leads to decreased pasture production and eventually desertification if nothing is done to correct these bad grazing practices. It's not the hoof action that is the problem, as this is actually very good for the grasses (provided that not too much "hoof-action" is done on the plants, turning the pasture into a mud-wallow), but the way that cattle tend to come back to a favorite spot over and over again. This leads to decreased root biomass in the grasses, and also exposes the soil to erosion.

Other times "they" need to move cattle is when it is time to process them for vaccinations, deworming, tagging, etc., and when cows need to be preg-checked and body condition scored prior to fall weaning and going onto winter pastures. Sometimes cattle need to be moved to a different pasture if there are major repairs that need to be done to a fenceline, or if a new permanent fenceline or water pipeline needs to be put in.