A few reasons:

- If you made a dietary change (e.g., more fiber and less saturated fat), the tests tell you whether it actually worked.

- Statins are dosed based on the LDL or ApoB reduction you need. The most powerful statins cut cholesterol by 50%, but at higher doses the risk of side effects does go up.

- Other medications like PCSK9 inhibitors and ezetimibe exist, and whether to use them depends on the other numbers. For example, PCSK9 inhibitors have a secondary benefit of lowering Lipoprotein(a), and statins and GLP-1s have a secondary benefit of lowering inflammation.

- If blood pressure, HbA1c, etc is high, then there's a slightly different set of diet advice and a very different set of medications. For example, lowering blood pressure through diet is mostly about reducing sodium and increasing potassium.