I have read in many post that in general is better to have a new job offer before quit, and many times this is due the HR team and company politics. I can't understand this.
The short version:
Most people never really leave high school.
The idea is just like in high school where someone wants a boyfriend/girlfriend that already has someone, but they overlook all the "single" people, regardless of their attributes, because, "Well, if they were worth having someone would already be with them."
The same thing happens in business. People believe that anyone worth having is already employed somewhere, and "stealing" them is the only way to get someone good, and that anyone who isn't employed must "have something wrong with them."
Now, this doesn't mean that everyone who is employed is worth hiring, nor does it mean that everyone who is not employed is overlooked potential, but to paint with such a broad brush is a sign of an underdeveloped emotional state. Yet, it happens all the time.