You are somehow right that if there would not be any scarcity, then you could just as well centrally plan all the needs and desires, produce them, and send them off to who-ever wanted them. 

But the problem is the allocation of resources to those actions that respond best to consumer's demand, all while knowing that they cannot be satisfied all of them.  So the question is not only: "what do consumers want ?", but also "what demands of consumers can we satisfy by using a limited and shared amount of resources".  And that's where competition comes in: I may know a way to satisfy a consumer's demand by using 3 resources A and 5 resources B ; now someone might come up with the idea to satisfy that same demand (or a very similar one) by using 4 resources A and 3 resources C.
First of all, a centrally planned economy may not know that someone had this idea.  Next, we don't know if it is a good idea: is it better to consume 3 resources A and 5 resources B or rather 4 resources A and 3 resources C ?  That will depend on how those resources are solicited by OTHER needs and by OTHER ideas by other people to do this.
So without knowing all these things, there's no way to find a good deal.  Moreover, why would someone make any EFFORT to try to find another way of producing, if there's no personal gain to it ? 

So in the end, if you are going to centrally plan the rewards  to people finding new ways of producing (taking risks !  It might not work) equal to the gain they allow to make, then you could just as well let them do their business in a free market, no ?

And if not, then these people will not be motivated to find, and even to tell, their invention of a new way of transforming resources into products.