Allow arguments/parameters to be passed to variable definitions, effectively creating user-defined functions.
For example, suppose that I want to define the function TagSum, which adds the values at all events in a time range. Then:
TagSum(tag, start, end) = TagMean(tag, start, end) * EventCount(tag, start, end)
Kenneth Barber commented
In the question linked below, Asle Frantzen brings up a good point that functions should be treated as first-class citizens. This would be useful to be able to define a function in a variable.