I am occasionally asked this question. The truth is I do not consider those exercises very important at all for new students looking for realistic self-defense fighting capability.
The San-Shin kata were never intended for real fighting; not now or not never.
No Japanese warrior ever fought using a kata from that set.
They were most probably created in fairly recent times based on historical fundamentals as a set of conditioning exercises that encourage growth in skill qualities. That history is what I was taught by my seniors in the 1970s.
As conditioning exercises, they do appear in our very advanced handgun defenses. We do not teach them “in a group” though, because our curriculum is more comprehensive and thoughtfully spread out over a set series of months of lessons.
The so-called Kihon Happo began in the 1960s as a sometimes-changing group of simple fight exchanges originally called the Hatsumi-ha no Kata, or “Hatsumi branch (of the Ryu) example lessons”. Supposedly Toshitsugu Takamatsu created them as something Masaaki Hatsumi could teach his students in the 1960s.
After I returned to America in the early 1980s, the Hatsumi-ha no Kata underwent a few tweaks in Japan and were then offered as the 8 techniques of the Kihon Happo. “Kihon Happo” translates as “collection of basic principles” but is literally written “8 ways of basics”. But no Japanese thinks of “8” when hearing “hap-po” – they know it means “comprehensive collection”. In a play on words, Masaaki Hatsumi allowed there to be 8 techniques, coincidentally. He enjoyed puns and word plays as a form of humor.
The Kihon Happo were so new, and communication within Masaaki Hatsumi’s organization was so challenging, that I did not know about them as a “thing” until a few years later. I had earlier created a curriculum based on the 5 elements of the mandala I had studied.
Some methods from the original Hatsumi-ha no Kata and the Kihon Happo are of course spread out through the To-Shin Do curriculum today. But for above mentioned reasons of effective education, they are not taught as a designated “lump of stuff.”