SQL Doubt - Count(*)-1 in the denominator

Hi All,

Had a quick question, do we know why we have count(*)-1 in the denominator when we calculate average days between orders for each customers in SQL?

select CustomerID
, cast(DATEDIFF(dd, min(OrderDate), max(OrderDate)) as decimal) / (count() - 1) as [Avg_Day]
from Orders
group by CustomerID
having count(
) > 1

Thanks,