What are these beams for in my loft?
I think they're there for stabilizing the brown beams/joists.  Typically, a loft is unused space with no floor.  Between the joists, there is insulation supported by drywall or other material depending on the age of the house (as in pic 2 and 3).  

Typically one would put in blocking between joists to stop horizontal flexing and torsion. 



Since an attic is usually unused space, I think someone took a short cut and just screwed the 2x4s to the tops of the joist. (Significant labour savings!).  Your flooring probably makes it unnecessary from a flexing point of view, but you still have a torsion problem (especially since you're now loading the joists).

When you go to make improvements,  block the joists properly, but I wouldn't worry in the short term.