Particularly in action and adventure scenes, the scoring usually relies heavily on pushing rhythms and lots of rhythmical activity. If you do not want to fall back to using percussion all the way through or even a drum kit in such sequences, you need to incorporate an “engine” (= a part that keeps playing eighths/triplets/sixteenths) into the music to keep it rolling.
Ostinatos work great but also just repeating notes on one or several instruments. The problem usually arises from the fact that in an orchestra only few instruments are capable of conveniently creating a steady pulse. Usually all woodwind and brass instruments can not be expected to play an active rhythmical pulse for a longer stretch of time, simply for breathing or stamina reasons. So the strings are usually the only section that can without much struggle play rhythmically active passages for longer times.
So naturally any “engine” would be best suited in the strings. However, you will most likely need most of the strings for other tasks. Violins are often needed for melodic lines while Basses and Celli are obviously often needed to provide the low end substance. The instruments left are the Violas which in their middle range sit very well to provide any pulsing part. This is the reason why in many (non percussion driven) action cues, we hear a pulse coming from the Violas. Listen to the opening of BATTLE OF THE HEROES by John Williams which is a classic example of an engine in the Violas.