Because showing up is half the battle.
You have to be there when a client is looking for a new product or a new service provider.
You have to be there when a client is looking for someone to do business with. Otherwise, how would he even know you exist?
You have to be there when your boss needs something done.
You have to be there when she is looking for someone to promote. Make sure she knows you are reliable by consistently showing up.
You have to be there when someone needs help in anything.
You have to show up (especially when it comes to execution). You have to be reliable.
But, believe it or not, not everyone who says he will show up actually does. Maybe it is because of fear, laziness, or some other excuse (valid or not). He fails to realize that by not showing up, he loses his chance at getting selected.
Lastly, in fulfilling your dream, whatever it may be, you have to show up. You cannot just say it and not go at it. You have to be there in the thick of things.
P.S. Still, showing up is just half the battle. The other half? Being remarkable in what you do.