You are correct that MSEP provides it's own lighting onboard the floats. Most building and lamp post lighting is shut off during them. I haven't seen Festival of Fantasy yet, but I can't imagine them needing these lights for it, since it's runs during the daytime.
In my amateur opinion, these lights are a result of the "lawyering." As I said in my earlier posts, I have memories of these lights being turned on at night during peak times (before/after Wishes and Spectromagic) when the hub was flooded with people. The reason I think it's "lawyering" is because other areas of the park have supplemental lighting too...
I recall seeing a number of units like these on the roofs of Frontierland and Liberty Square buildings at peak times. Workers would have to manually pivot the arm into an upright position (like in the picture) and then turned on one by one. When done for the night, they'd be lowered back down and out of the sightline. They probably also exist on the roofs of buildings in the other lands, but I have yet to see them.
Perhaps someone more in the know can weigh in?