A spot in the white of my eye, looks like a freckle. Eye doc said no need to remove it. What is it anyway, and why did it develop? Had it for 10 years

Conjunctival nevus. The lesion is likely a conjunctival nevus but it could be a number of other lesions. It is the equivalent of a freckle on the eye but it can be removed if cosmetically undesirable.
Nevus. You have a small pigmented nevus on the white of the eye. This is just pigment cells that have normally divided and done this enough so that you can see it in the mirror. These are invariably benign, no need to remove it is true, but they can be removed as a cosmetic issue usually in an ophthalmologist's office. These rarely ever become a real problem.