my guess is the abundance of vitamin b12 and choline. You can get bio available sources of each without eating offal but it just so happens that beef liver is a high source of both and the chances of you becoming deficient in either are lessened if you eat it.
Linus Pauling Institute at Oregon State University
Top 10 Foods Highest in Vitamin B12 (Cobalamin)
As a side note, bone broth and bone marrow also contain high levels of b12. The cheapest source of choline comes from soybean (soy lecithin). There might be other reasons for eating offal but those are the two I can think of off hand. Currently, I don't see any reason why a person couldn't benefit from just using a supplement for the two but I'm not sure how efficiently those vitamins are absorbed in pill form as opposed to the natural sources.