First, it is important to understand some of the differences between IK and FK. When you are animating an arm in FK, you first move the shoulder, then the elbow, and then the wrists. That's more or less what we do when we move our own arms. So, all and all, FK seems a bit more similar to the way we move, with the movement starting on the shoulder. The process of animating an IK arm is very different; instead of starting the movement from the shoulder, you will work in the inverse direction. You position the wrist wherever you want it to go, and the rest of the arm kind of goes with it. Both ways of animating are perfectly fine, and, for example, when animating a character dancing, walking, or running, you can certainly achieve good results with both IK and FK.