Scarcity, Choice & Opportunity Cost

Two reasons it appears so often on the syllabus:

• It is the right way to evaluate decisions. A decision that "saves money" can still be a bad decision if the next-best alternative was worth even more. Cheaper flights save £60 if you can work that day, but they cost £130 of lost income if you cannot.
• It applies everywhere. Microeconomics (a consumer's spending, a firm's output choice), macroeconomics (a government's budget priorities) and international trade (which goods a country specialises in) all rest on the same idea.