The purpose of the difficulty is to maintain a 10 minutes average delay between each block and by the way to ensure the Bitcoin emission rate.
The difficulty is adjusted every 2016 Blocks (2 weeks) to compensate the increase or the decrease of the network calculation power.
If the difficulty is so low that the network generates blocks in less than 5 seconds this would be highly unsecure because this would not let time for the blocks to be shared on the network. The result would be a lot of orphan blocks.
Some altcoins were launched with 15 seconds blocks target. Many of them like smallChange failled and doesn't exists anymore.