We learn that our mistakes can be the cause of change. In effect, we look to disperse the energy with rolls, falls, or redirection, while becoming aware of what we can do to become the nage.
Something I call, "Get the hell out of the way, you are in the wrong place at the wrong time" when being uke and the inevitable technique is performed.
The awareness of turning the tables, making an attack less of an attack, or having the consciousness to slow down physical movements so the mind and body can react are all benefits of learning to give energy, and recieve energy.
So, indeed, ukemi can be as valuable as the offensive/defensive role as nage.