A technique is a fixed form in a fixed environment to reach a fixed goal. If you want to apply a technique in the real world, you have to adjust it because the environment is changing in the real world. In your life the goal can also change so you have to act and not to apply a technique. Learning techniques helps you to act better in your real life.
In order to practise a technique in Aikido, you normally apply it to a Uke. Then you tend to react to what Uke does and cannot perform leading. That is why it is difficult to develop a correct Aikido technique. In order to lead Uke, you have to imagine the future of one second during the technique. The easiest way is to do the technique alone imagining a Uke.
However, it is not easy to imagine especially when you are a beginner. You have to develop an ability of imagination.
Exercise or Undo is an activity to develop your ability of doing something or to keep health.
Kenko Taiso is for keeping health and Aiki Taiso is to develop the ability to imagine Aikido techniques.
When you apply a technique to Uke, both change of form and change of place are happening all the time. Is is difficult to imagine the whole. So you have to imagine separately the change of form and change of place. In classic music they did it by separating rhythm and melody.
Aiki Taiso is different from performing technique by yourself. When you perform a technique by yourself, you change form and place at the same time with the imagination.
When you do Aiki Taiso, you first change your form without changing place. Then after you change form without changing place, you change place without changing form. Sometimes you change form, change place, and again change form.
By practising Exercises or Undo, you develop the ability of imagination of Aikido technique with Uke.