One of the main responsibilities for an accountant or bookkeeper is to properly account for all expenses in the period in which they are working. Payroll is a major expense for most companies, and getting it wrong can result in major accounting problems.
Most companies report expenses monthly, and then summarize them quarterly and for the year. Some expense, such as rent, may fit nicely in the books since they are paid each month. Others, such as supplies, are ordered when needed, and must be accrued properly.