Kidneys, liver and offal in general have high levels of essential vitamin B6 which is involved in the metabolism of protein and is also essential for the formation of red blood cells, plus antibodies and the brain chemicals called neurotransmitters - no brain = no hope of enjoying the next plate of tripe.....
Plus several other vitmains are present in good quantities - Vit A, B1, B2, B5 which keeps the nervous system and skin healthy (use the liver/kidneys in a casserole or pie as frying or high temp dry heat easily destroys B5).
Other reasons? Liver is high in Vitamin D - one of the few sources rich in this one, which absorbs calcium from the intestine (ie from other food stuffs) and so can help to prevent brittle bone disease or osteoporosis. Vit d works with calcium to develop good bones in young people for strength of development.
Vit K - helps the blood to clot after wounding, sp a pretty useful one - but the body does make some of its own from the happy gut bacteria (don't put that one on the pro-biotic yoghurt pots do thye?) , so not really needed to take in too much from other sources. It also is being investigated as a possible aid in developing bone growth and helping in reducing deaths from coronary heart disease as it seems to have an effect on taking away calcium growths form bllod vessels - so it could help you live longer....
Hope this is what you were looking for..