Data in FileMaker calculations maintains its type even when it isn't coming from a field with a particular type. It is sometimes helpful to know the type of a given piece of data to know how to process it, but right now we only have built-in functions for detecting field data type. The function would accept any data as input, and return either a number representing the type (e.g. 0 = number, 1 = text, 2 = time, etc.) or text describing the type (e.g. "number", "text", "time", etc.). I'd personally prefer the former for localization and future-proofing reasons.
Detecting the type of arbitrary data in a calculation is already possible, but cumbersome (see the # ( name ; value ) custom function).