Some calf raisers will declare the necessity of using a bottle (or a bucket with a nipple) so that calves get to experience the full extent of esophageal groove reflex.
As a reminder, this groove is a muscle at the calf’s throat that curves when suckling and milk protein is ingested. It goes all the way to abomasum (true stomach). Without it, milk would sour in the rumen, causing acidosis, incomplete milk digestion, and many health problems that can lead from diarrhea to death very rapidly. The esophageal group is not affected by the calf’s posture, but rather by its desire to suckle and the presence of milk protein in its pharynx. Thus, it is an involuntary muscle movement. It also depends on the slow suckling of milk because large quantities of any liquid will cause an overflow of liquid into the rumen. These are then the reasons why some calf raisers prefer a bottle or a bucket with a nipple.
In contrast, thousands of calves are raised by drinking milk or milk replacer liquid from a bucket. By avoiding large meal portions, and frequent meals, without letting the calves go hungry between meals (by offering free access to a high-quality starter dry feed), bucket feeding can be as successful as bottle feeding.
The main advantage of bucket feeding is its easier handling. It requires simpler and faster cleaning procedures, it can be filled on the spot (an advantage for large farms), and it allows a more drastic change to milk replacer solids according to seasonal needs. In addition, it gets the calf accustomed to bringing its head into an enclosed container, the same where dry feed is located next to its milk bucket. This alone, in my opinion, is important enough as it allows a more rapid growth, as the calf converts to a ruminant faster, allowing earlier weaning.