Consider these advantages when deciding if you want to use a custom function:
Custom functions can replace sections of formula logic common to many different formulas that, in turn, may be used in many different reports.
Custom function can reduce the need for your users to write formulas themselves; you can provide them with the formula logic they need in a custom function.
Custom functions can have arguments, which makes them flexible.
You can define help text and default argument values for custom functions, which makes it easier for your users to use them.
Custom functions, unlike User Function Libraries (UFLs) to which they are related, are saved as part of the report file (.rpt) and don't have any external dependencies.
Custom functions are stateless; the same input always yields the same result.