I am totally convinced that, in most cases, it is good for students to understand why mathematical rules work. I could write a very long text justifying this belief but, from an academic point of view, my personal opinion has no value. I need academic articles/books written by researchers of mathematical education supporting this idea.
Is there an agreed upon goal of mathematics education? It seems that you are assuming (correct me if I am wrong) that there is a "real goal" of mathematics education, and that "understanding the reasoning" is not one of those goals. Then your question is for evidence "understanding the reasoning" supports the "real goal".
I do not view things this way. I think that everyone has their own individual motivations for how they choose to engage (or not engage) with mathematics, and they are all equally valid. I have personal beliefs that engaging with mathematics in particular ways can enrich someones life, so I have built a career around sharing that perspective. I know that one's life can be enriched in this way because my own life has been so enriched. No academic justification needed.
For an analogy, I find it very enriching to forage for wild foods. When I am hiking and eating a pawpaw, and someone asks me what that is, I do not need a research study to justify sharing my enthusiasm for my hobby.
Now there are interesting math education research questions you could ask which are related to this question. It would be interesting to know the answer to questions like "Does having an explanation of a rule increase the chance that the rule will be applied correctly?". This is a question about objective reality, and hence has a chance at being answered by research. The value judgement that this would be a "good thing" is not answerable by research.