Usually those "you are a cheater" things are made by people who crack or hack games, the cheat codes for DS1 and LOA were left in the game by the original game developers, you might ask why did they do that?, I don't know the answer to that and I suspect no one else does either.
Those codes are generally useful to mod developers, and DS was alwaya a mod-friendly game.
You had the ability to set a brand new character to match the area of map you were testing, without having to build him up through the preceding zones, for example. There are codes to change the size of the character, so you can check the clearance for doorways, passages etc. as well as thing like toggles for UI elements that might obscure items you're testing. Making a character invulnerable is also useful when you're testing new animations for the monster attacks, as you can them play over and over without having to fight back. And so on ...
You had the ability to set a brand new character to match the area of map you were testing, without having to build him up through the preceding zones, for example. There are codes to change the size of the character, so you can check the clearance for doorways, passages etc. as well as thing like toggles for UI elements that might obscure items you're testing. Making a character invulnerable is also useful when you're testing new animations for the monster attacks, as you can them play over and over without having to fight back. And so on ...