Can anyone explain why I would use Subtypes as opposed to just have a symbology Representation set? I understand Subtypes categorize the data so when brought into a Map, it is represented as unique values according to the subtyped field but seems like Representations do the same thing?
Subtypes are typically used to ensure data validity.
Representations, on the other hand, are specifically employed in displaying data. Representations allow more display options than the standard symbology, and in particular are helpful in that they allow one to display data in different ways without altering the source data. For example, if one had two coincident lines in GIS to display, like bus routes on the same road, with Representations one could show them side-by-side for reader clarity instead of overlapping each other. And the original data would not need to be altered to do this.
Chris Donohue, GISP
There are some other nice benefits of subtypes include seeing the subtypes in the preview window in ArcCatalog
and a way to classify/group features "permanent" in your dataset...similar to domains, but unlike domains that are one per field, a subtype is one per dataset (and the value must be numeric)