The metal insert would have been a "bodkin" used as a crossbar to limit blade penetration.
Notice the sword all the way on the right (yes, all three shown in illustration are specialized hunting swords):
This was particularly useful when you wanted to be able to control dangerous game; to keep them at a distance rather then having them ride up the blade to within striking range. It doesn't make much sense on a knife; although, these daggers evolved from the hunting plug bayonet, so it may have been a stylistic element retained from the earlier form.
n2s