"/execute if function" cannot be run without further subcommands

Normally, any "execute if" statement can be run on its own as a test which returns a result value. For example:

/execute if dimension minecraft:overworld

This is a valid command on its own and doesn't require any further arguments - the command simply returns 1 if run from the overworld and 0 otherwise. All other forms of "execute if ___" follow this same format, except "execute if function". Trying to run execute if function on its own gives an error message, and it only works if you add further subcommands.