I work in a team of 5 including the manager, 3 of us are full time and 2 are 3.5 and 4 days respectively. That means we are a maximum of 4 on a Monday and on Friday afternoon, 5 on Tuesday, Wednesday andThursday, and 3 on Friday afternoon.
Depending on which day of the week and which people, if 2 people are off that can leave us with 2 people or even 1 (Friday afternoon). Add in sick leave, fit round the school holidays etc, and potential leave dates can be very prescriptive. So in our case "no cover" is a very present reason for refusal of leave.