It's very useful (essential) if you want to easily display your results spatially, i.e. using terrain data (DEMs) to develop inundation maps. The georeferencing doesn't affect computations, but if you've constructed your model using GIS tools, your channel centerline and overbank distances are calculated automatically. You can georeference an existing model, but it just contorts the display of your model schematic to the georeferenced location, and doesn't do anything to the underlying model data. If you have terrain data you can then compare your georeferenced cross section geometry to the terrain data, which can be useful.