Subject: During evolution why do animals lose bodyparts they dont need ?

It is understandable that animals with better adaptaion stays and others 
disappear. But what I dont understand is the fact that the animals having 
a better adaptation if they are not used are losing out ( like dodo losing 
its power of flight). It doesnt hurt a dodo if it has the ability of 
flight ( or some other example). Why do they lose out ? There shouldnt 
both ( with and without that adaptaion) live in harmony ?

Re: During evolution why do animals lose bodyparts they dont need ?