
So, where was I when the storm hit?
Ah, yes, practicality and seppuku.
Because, while there was, over time, a lot of formality developed around the proper behavior of the buke class, all of the actual prescriptions you can find in the house codes and law codes are extremely practical.
Seppuku as execution was, itself, immensely practical. Allowing the executed man to 'clear his name' that way meant that his family and belongings then were not tainted. The lives (and labor) of the family were retained, and the belongings were 'clean' and could be either left or confiscated, whichever suited the lord in question better.
There's also the basic fact that, in a world ruled by force of arms, any lord wants to be absolutely, positively, sure that the retainers around him were loyal to him, trustworthy, and not going to go give aid to his neighbors. If either of the first two failed, the only real way to prevent the third was to kill the man.
On the battlefield, of course, there was another aspect to it. Torture, all through these periods and well into Meiji in practice, was a popular and accepted way to get confessions of crime or information from captives. Given that buke put such a premium on being all stoic, there's a double problem with being tortured: one, the basic fear of pain and of betraying your own side and two, the shame of being driven to speak by pain of the body which one is supposed to hold as illusory and negligible.
And, then, of course, there's the collective responsibility issue. By ending in seppuku, a warrior makes sure that his family won't be blamed or punished for his failure on the field.
Now, there were also one or two references that indicated it was more common to let a defeated lord live and keep his lands in return for an oath of allegiance, than it was to toss him out and put in someone else. I strongly suspect that this socio-psychological crux is actually where the trope we see over and over in sports anime comes from: that there's no shame in losing if one has done one's best to win (honorably). That is exactly the sort of mental out that would be required for the defeated not to fester in shame and resentment and rebel later.
During the Muromachi period, specifically, I suspect that this psychological trope was starting to gather momentum, and was jostling with the /other/ common trope that loss was no shame if you ended as your own man (with your belly slit open). One of my print sources is very clear on the individualism inherent in the samurai codes, and that seppuku, in particular, was a powerful expression of that--of keeping, to the very last, personal control over one's integrity and never giving it up into other hands.
The 'okay to lose if you tried' is actually a far more collectivist approach, one that retains all members of the community and allows them to take pride in /whatever/ their place ends up being. Even if that place is second or third or so on.
The 'okay to lose if you die before they take you' is, by contrast, intensely individualistic and even selfish. It removes the person from the pool of lives and hands available to keep the whole community (state, nation) together and safe and functioning. It's really no wonder the Tokugawa worked so damn hard to break the buke away from that ethos.