Any builders out there ??

That is where hot air escapes more easily hence the radiator provides a pocket of hot air to diminish it.

Aren't forced air vents usually under windows? My apartment had electric heat and the registers were under windows as well.
Maybe because it's a source of cold air? Or maybe people are less likely to put big furniture in front of windows?
I've often wondered this myself actually.

They complement each other as temporary shelves when cleaning windows or working on the radiator? Provides an access for easily venting excess steam outside? Goes with the floor plan?

"Floor draft is directed upward and warmed as a result. Thus creating consistently warm circulating air"
So says a builder.
Things you learn eh ??

That's where the cold air comes from...