The effect is in all likelihood the same as with dynamic exercise. An increase in muscle fiber and diameter and an increase in capillarization. That's for the well-trained person. If a novice to doing strength exercise stars with wall squats (or any type of squats) the body's first response is to improve the the efficiency of neuromuscular activiation. Consider it a "doing better with what you have" strategy. Once that is optimized, the stimulus elicited by exercise that pushes the limits of performance is to increase muscle mass. So, yes, holding for as long as you can is the best strategy, because it pushes that limit. And you'll notice how you can keep the position longer and longer.

And yes, it works very well with the plank, too. When I started my wife on doing the plank (she got motivated to include the plank into her strength training routine, because it's an excellent exercise for stomach muscle tone), she could barely hold it for 2 minutes. After a few weeks, she was up to 8 minutes.