3 ideas
The self is pictured as a kingdom: the heart (qalb) is the king, the intellect is the vizier, and the body's faculties are the servants. Appetite is like a tax collector and anger is like a police chief. Well-being depends on this order being right, so that intellect advises, the heart rules, and appetite and anger serve instead of rule. When the servants take over the kingdom, the person is ruined, however successful he looks from outside.
Every person holds four natures: the animal that eats and lusts, the predator that attacks and dominates, the devil that schemes and deceives, and the angel that seeks knowledge and contemplation. To know yourself is to ask which of these your time and effort actually serve. Happiness belongs to the angelic nature, which lives on knowledge and nearness to the divine. The other three natures are tools to be tamed, not masters to be fed.
Al-Ghazali builds on the saying 'He who knows himself knows his Lord.' He argues that the soul is an unseen, unified ruler that governs the body without being located in any one part of it. That makes it the closest available analogy for how God relates to the world. So inner self-examination is not a detour from theology; it is the first and necessary step toward it.