I would like to note the difference between the Seal as an image, the Seal as an object and the Seal as a force. What we see in the books is just an image, a sigil, which, nevertheless, has a realizing power. The original Seal Item has this power to a much greater extent, and the Keeper embodies its very Principle.
It is clear that considering the totality of 72 Names of Geniuses as Shemhamforash, the magicians hoped to gain absolute power. Therefore, the Keepers of the Seals were exposed to dangers not only from the "Demons", but also, to a greater extent, from the megalomaniac magicians.
The first 72 Guardians, having taken their Seals, dispersed all over the world, and lost all contact with each other. This was done deliberately for security purposes, to avoid the possibility of access to other Guardians through any of them, since combining the power of several seals could do as much harm as good.