When it comes to hand to hand combat, great reactions are a must. It is not just pure reaction time that betters, but also anticipation and efficiency. If someone looks like they are about to strike, knowing that they are about to strike will give a much greater chance of evading it than reacting when they are striking.
In combat sports, one must also know how to react while rocked by a hit. Getting rocked will daze you and essentially make your mind temporarily stop functioning. One needs to be able to recover quickly and know what to do next to evade more damage. Not doing so can be devastating.