Low guidance lets the model wander: prettier, looser, less like what you asked for. High guidance forces compliance: closer to the prompt, and past a threshold, burnt contrast, over-saturated colour and a plasticky surface. The useful range is narrower than the slider suggests.
Practically it is a diagnostic. If a frame keeps ignoring one element of the brief, raising guidance is the wrong first move: rewriting the prompt so that element leads the sentence is usually the right one. Guidance is for fine adjustment, not for arguing with the model.
What CFG scale should I use?
Start in the middle of whatever the model documents as its default band and move in small steps. If you need to go far above it to get compliance, the prompt is the problem.
Why do my images look burnt or over-saturated?
Guidance set too high is the most common cause. Drop it, and move the emphasis into the prompt wording instead.