Why do side planks if normal planks engage the entire core?
First it's a good thing that you are concern about your fitness.
See, Exercises are of different type and you can't compare one by other.
Yes, it's true that normal planks engage entire core and there are number of benefits of doing this exercise.
In fact, Plank is one of the best exercise for abs, back and core. But it doesn't mean that you won't practice other exercises which same impact.
Side Planks have some additional benefits, I would mention some of these :-