Aiki Taiso is for developing Ki. Doing Aiki Taiso coordinates the nervous system (network of electrical signals) with the Facia ( network of mechanical signals) using the Mind (cognitive features of the brain). As the coordination progresses the cognitive features are used less and less (relaxation).
Indeed, aiki taiso is used as a means to practise mind-body coordination while moving (it is all too easy to do it while standing or sitting). During classes, various tests are used to ensure coordination and relaxation throughout the movements.
Thus, they are not mere physical movements, and there is little point in mimicking them off videos or books. One must also incorporate the mental element, which may consist of a visualisation of extending ki or allowing weight to settle. Ultimately, they become a feeling which one taps into.
Very importantly, aiki taiso should be seen as components of aikido movements. By learning to perform the various aiki taiso exercises - with the right feeling - aikido movements can become more relaxed, minimising effort and improving effectiveness.
For example Ude Mawashi Undo (Arm Swinging exercise) is a practice of dropping the arms using only gravity (ie not pushing down or not holding up). When applied in aikido movements like ikkyo, kokyunage or iriminage, these techniques can be quite powerful. Observe Koichi Tohei using the principle extensively in his throws.