No particular reason. I was just showing you an alternative.
I already explained that. But I'll try again....
Start with
Entity["GeographicRegion", "Europe"]["Countries"]["Flag"]
We first evaluate
Entity["GeographicRegion", "Europe"]
which just returns the same expression, because it's just an "object" that represents a particular semantic. That particular "object" can be applied to an argument, and in this case it is so applied, so we next evaluate
Entity["GeographicRegion", "Europe"]["Countries"]
which gives {[Entity["Country", "Albania"], ..., Entity["Country", "VaticanCity"]}
. Presumably you expected this.
But there is another step remaining in computing the expression. We apply the result we have so far to "Flag".
ExpressionWeHaveSoFar["Flag"]
The ExpressionWeHaveSoFar
is {[Entity["Country", "Albania"], ..., Entity["Country", "VaticanCity"]}
. so we really have
{[Entity["Country", "Albania"], ..., Entity["Country", "VaticanCity"]}["Flag"]
At this point there is nothing to do. We don't have any rule for evaluating this. So, that's the end of the computation.