There are some circumstance you should consider anodizing after CNC machining：

Corrosion Resistance: When corrosion resistance is critical, such as in marine environments, outdoor applications, or industrial settings with exposure to harsh chemicals.  

Wear Resistance: When enhanced wear resistance is required, such as for components subject to abrasion, scratching, or sliding contact.  

Aesthetic Appeal: When a specific aesthetic appearance is desired, including clear, dyed, or hard-coat finishes to enhance the visual appeal of the product.  

Electrical Insulation: When electrical insulation is needed to prevent electrical current flow or to improve the electrical performance of the component.  

Improved Adhesion: When enhancing the adhesion of paints, adhesives, or coatings is necessary for subsequent finishing or assembly processes.  

Surface Protection: When a durable and protective surface layer is required to safeguard the underlying aluminum from damage during handling, transportation, or storage.