I think that when most people look at two people practicing aikido they see one person providing some sort of attack so that the other person can practice an aikido technique. Then the two people switch roles, so both people get to practice aikido. By this view, everyone wastes half of their class time providing an attack so that someone else can learn aikido. Unfortunately, this is what is happening in some aikido dojos.
It should be that people performing both roles are practicing aikido, albeit different aspects of aikido. The label we use to identify the person providing the attack is uke. This means receiver, which, on the face of it, seems quite strange. However, from an aikido perspective, the role of uke is to attack the other guy, who we call nage or thrower, and then to “receive” his or her own energy as nage redirects it back to uke. We call this deliberate and skillful process of receiving energy "taking ukemi". Of course, since nage is receiving the energy of uke’s attack and redirecting it back to uke, people in both roles are supposed to be learning to receive and redirect energy being thrown at them.
Don’t be confused by my use of the word energy. I am not talking about some mystical Star Wars type of Force. I am talking about the physical energy imparted upon the impact or collision of two bodies, or, in these situations, body parts. The idea is for nage to avoid being struck by uke’s attacking energy and then for uke to avoid being struck by nage’s return of uke’s energy to its source. This is not easy for people in either role to learn to do, so it is a good thing that an aikido practitioner is practicing this during the entire class, not just half of it.