The same aggregate formula can work within different groupings in different reports. So, design your formula correctly and then use it freely in various kinds of reports.
Remember, formula columns are additional columns in your data table. However, aggregate formulas do not appear in the data table but are tied to it.
That being said, aggregate formulas must actually contain an aggregate function of some sort. Conversely, formula columns cannot contain aggregate functions.
You can, however, combine them both when creating Aggregate Formulas.
Aggregate formulas are often the much more efficient way to build the report you need.