To explain why the Triple Burner Back-Shu point is next to the Kidney's we need to explore the nature of the Triple Burner. The Triple Burner's nature has occupied Chinese doctors for centuries and there are different views of it; as usual in Chinese medicine, different view are contrasting but not necessarily mutually-exclusive ones. For centuries, Chinese doctors have been discussing whether the Triple Burner "has a form" (you xing) or not. The Chinese expression "has a form" means that the Triple Burner is an actual Fu organ; if the Triple Burner "has no form", then it is not like the other Fu but just a complex of functions "without form". Both these views are correct!
Photo by Shutterstock 281440808. Image of an acupuncturist pointing at BL17 on an acupuncture model sourced from Shutterstock. Available here.