Let's go outside of food just for a moment to answer this question.
Let's talk about fashion; like a tailored shirt or dress.
You get to select the material, how it fits; (loose and comfortable or form fitting), the exact color and print you desire, etc. it perfectly contours to your body and it perfect for the occasion you're purchasing it for. No one else will be wearing this item of clothing.
Catering is similar in that we're creating a menu precisely tailored to your specifications. When your chef shops, it's not in bulk to get the cheapest price. Chefs shop to get the best available fresh produce and protein sourced locally and seasonally just for your event. Many times a chef will add to your menu based on seasonal ingredients that she or he happened upon when shopping.
Such was the case, when I catered a multiple day event for family vacationing in the Disney area.
My Croissant Encrusted Brie usually has a Blueberry or Strawberry Preserve made from scratch. I happen to run across the sweetest nectarines while shopping. I created a fantastic Nectarine Compote and it was phenomenal. The family raved about how good it was and that it was not part of the agreed upon menu.
This a similar philosophy used in upscale restaurants offering their "off menu" Amuse-Bouche.
