It is top-down, so why bother with wallhack
The camera shows the room you are standing in; what is happening behind the wall of the next house stays hidden. Highlighting removes exactly that blind spot, so you know how many zombies are inside before you open the door.
Any use for finding vehicles
Transport in HumanitZ is scattered across the map and usually broken. Highlighting shows where a car stands and what condition it is in, so you avoid walking half the map for an empty shell.
Is there a godmode
There is, and in this game it changes more than anything else: hunger, thirst, infection and a zombie crowd kill quickly, and there are no saves in the usual sense. Turning damage off turns a run into calm resource gathering.
How does co-op play out
The build installs per person and highlights only for you. A friend playing without it will simply see you leading the group with confidence. The host checks nothing, since the game carries no anti-cheat.
So there is no risk at all
On your own and on private sessions there is practically none, because nobody is checking. The only way to spoil things is playing with strangers who notice the hints and kick you from the lobby.
Why is it so cheap
The 4 to 22 dollar range comes from the game having no protection: the build's author does not need to get around drivers or hide a process. The real work here is keeping up with updates.
What does Arcane do beyond highlighting
It separates loot by category, shows zombies at distance, provides godmode and speed adjustments. Everything toggles on hotkeys, so anything you do not want can stay off.