There are definitely some reasons to keep the fields clear of weeds — not only do they compete with the plants for water, sunlight, nutrients, etc but weeds can cause issues with crop quality as well as yield. Damage from animals is usually not a big enough problem to have farmers focus on doing something about that but if an area has had a lot of land use changes like subdivisions going in, you can have trouble with food supply for wildlife that may lead to increased eating…. sadly that means wildlife will likely die of hunger at some point as most crops are usually only feed in one season.