Good question!
I think sardines are partially rotting (digested) like fermentation ,like a dog will take a piece of raw meat and partially burries it in the sun and watches over it while it rots ,waiting for it to sweetin up allowing those tiny little friendly (to a dog) bacteria to help digest it and allow it to be much more beneficial and digestible!
Sardines remind me of when you gut a fish and the gut is filled with small fish all soft and pliable like a sardine(little fish)! One of the Guiness book of records oldest men 120 I beleive, ate nothing but sardines and pickled hard boiled eggs, basically predigested foods!
I think these partially digested foods have many benefits without using much our bodies energy to assimilate them!
Or something like that,hope this makes sense to others!