Rosemary Beetle

Why is that that some of the most attractive looking insects are also considered garden pests?  This is a rosemary beetle discovered on some of my lavender today.  It eats the leaves of aromatic plants, however according to the RHS it can be accepted as part of the life a healthy garden as it does not eat enough to harm healthy plants. So once I'd finished taking its photograph, I left it alone to carry munching.