I know some people may find it unnecessary to have multiple liveries of the same cars, but personally I think it's good because it provides more variety of cars during offline racing. Even Gran Turismo has been doing this since GT4. Sadly, this practice has been disappearing in GT Sport, but it is still present in cars like the BMW M6 GT3 Walkenhorst Motorsport '16 and BMW M6 GT3 M Power Livery '16.
Another reason to have multiple actual liveries of the same cars is because it would be more exciting for fans of those series to see their favorite teams featured in Gran Turismo. For example, as a WEC fan, I would be very happy to see teams like United Autosports, JOTA, Team WRT, Dragonspeed, AF Corse and TF Sport in GT7 with oficially licensed liveries
Although if I had to choose a specific livery for the Holden Commodore and Ford Mustang, I would choose Triple Eight Race Engineering and Dick Johnson Racing for being the two most successful teams in recent years.