Sure, insulin levels when too low cause certain measureable and identifiable changes and replacement corrects or controls them. Lots of diabetics, though, are not too low in insulin, but do not use it properly at the cellular level and some actually have elevated levels.
With respect to testosterone, some men are clearly abnormally low and benfit from testosterone. However, what is abnormal is not clear as there is a large range of normal levels. Use of testosterone has some dangers. It is not a benign thing to use.
I agree, but that doesn't mean it shouldn't be used.