There are so many reasons, actually. Here is one:
I find the individual asanas fascinating and fun. It can be meditative and illuminating to break down a pose (by anatomical focus, actions, alignment, variations, symbolism, etc.) and practice it in a static way.
But it is also important to learn how to MOVE in a healthy and sustainable way, a skill that really serves us in daily life. Moving in between the poses with attention and care teaches us about our biomechanics. Synching our transitions with the natural actions of our breath and vital life force, helps us set ourselves up to be efficient and open in the asana with a balance of effort and relaxation. This, to me, is the gift of Vinyasa Yoga.
And what if we could apply this principle to life? To be able to hold ourselves, understand ourselves, move freely, and flow into and out of transitions gracefully, requires practice. That's what our mats are for.