"Report aircraft type" is mainly used on arriving ATIS's so that the aircraft actually operating the flight can be seen to match the FPL type. This can make a lot of difference to the ATCO putting a/c together on Approach. For example out here in ME we have lots of airlines who operate Heavy, Medium and even Light aircraft on same routes.
So a late a/c swap could mean, if not noticed, the wrong vortex gap being given on final approach and all the possibilities this could create!!