My grandfather's desk had the best light in the room.
I think the bolded phrase means— desk possessed the best light in that room. (Am I correct?)
Merriam-Webster has this definition of "have" that applies:
3 : to stand in a certain relationship to
we will have the wind at our backs
In that example sentence, the people speaking certainly don't possess the wind at their backs, but have a certain relationship to it in the same way the desk has a certain relationship to the light.
"My grandfather's desk had the best light in the room."
Translates as: The best PLACE for light (say for reading) would be at "my grandfather's desk".
Implies the question: "where is the best light" which may or may not have been explicitly stated. For example, "I need a place to write" implies a physical need for light (writing in the dark is usually not a good plan). "Has" is used in the same was as "Joe's Pub has the best beer".
In this meaning, "my grandfather's desk" possesses "the best light" whether it is sunshine, an electric light or some other form of light. And, if it is light -- or beer -- that I want, then "my grandfather's desk" or "Joe's Pub" are the places to find what I want.
my grandfather's desk
.