D&D Alignments - How to Use Them!

Alignment describes the setting. There are forces of good and evil, and these two forces are opposed. There are forces of law and chaos, and these two forces are opposed. Your character's alignment may have been defined by the rules as a stance or an attitude, and is often treated like a personality type by players, but your character's alignment instead describes their alignment to these forces, or to neutrality if they're unaligned.

What is good? What is evil? Are these ideas up for debate? The higher powers in the setting (gods for example) decide. In other words, it's up to the GM to define these concepts for their setting as s/he is the designer and curator of the setting, and is the one who portrays these NPC higher powers. What if your character has the desire to be good, but lacks the knowledge of right and wrong or the willpower to act on it? That's also up to the higher powers of the setting.

If your setting does not contain these forces, then why use these alignments?