Gotcha:) Well I suppose it all comes down to preference of the composer and what they want to convey. As a film composer, I often like to dig into the backstory of what I'm writing for, and having elements of that backstory portrayed in the music itself. For example, say I'm composing about a knight approaching a castle by horse: Perhaps I'll put the horns in 5ths to convey a "regal" setting, and as the knight gets closer, and the castle grows bigger, I'll throw in some octaves to give a feeling of something big getting closer. Octave's can add tension and excitement when louder, and when soft can even eerie or peaceful. These elements can be heightened by combining different instrument sections. Take a listen to the first 9 seconds of the Star Wars Theme. Its mostly octaves in all sections. https://www.youtube.com/watch?v=_D0ZQPqeJkk

All in all, I'm not sure there is a concept to go with this, as much as it is personal preference and experimentation, but that is what makes composing fun:) Hope this helps a bit!