The "helmet covers" are also known as skull caps or jimmy caps. They are used at most football practices at any level. They are used to distinguish one team from the other during practice. It is easier to slide a skull cap on or off as practice goes on rather than pull a jersey over shoulder pads
The bills mostly use them during special teams practice when a team may consist of offensive (white jersey) and defensive (blue) players on the same team
Most high school programs play players on both offense and defense, and they just use one standard color jersey for practice. In this case skull caps are very important