Even mental rehearsal of techniques can be a good form of solo training. Many Aikido movements are derived from classical weapons arts so there is a historical reasoning for learning weapons movements, and weapons training is helpful for learning proper distancing. As well as this, many advanced Aikido techniques and kata movements involve defences against weapons. To ensure that these techniques are practised safely, it is important for students to know how to attack with weapons, and how to defend against these attacks. Being attacked by a weapon can give more realism to the techniques being practised.
Weapons training is also an excellent way to learn the principles of lines of attack and defense.