When the whole is greater than the sum of the parts. This is one reason why we choose movements like squats, deadlifts, and presses over leg extensions, hamstring curls, and lateral raises. In nutrition this is why we prefer people eat mostly whole food sources to get their vitamins and minerals. Breaking things down into their constituent parts always misses something. Zooming in too far misses the bigger picture. Knowing what every vitamin and mineral does is cool, but not nearly as important as remembering to consume lots of fruits and vegetables, whole grains, and lean proteins.
If you have a specific deficiency it might be important to supplement (something you’ll know because your doctor will have diagnosed it). If you’re someone with more specific training goals (e.g. bodybuilding) then you’ll likely need more isolation exercises. But for most people most of the time, focusing on squats, deadlifts, and presses while eating a diet rich in fruits and vegetables, whole grains, and lean protein will provide more clarity than trying to figure out how to strengthen each of the 650 muscles in the human body individually.