Weapons training builds strength, coordination, manual dexterity, agility and superior motor skills. Historically, taijiquan was not designed for unarmed combat. Weapons came first; namely a stick, swords and knife.
In real life your opponent is more likely to be armed with a knife or a stick. The ability to handle such weapons with dexterity and skill will train you to be able to fight against them. You will understand their strengths and weaknesses, range and power.
Weapons training also works the hands, wrists, arms and back in a way that qigong and the Long Yang form fail to do. Your arms will become very strong but not tense.
The final benefit is 'connection'... being connected within your own body is essential, but being able to extend your strength out into the opponent is vital. To use a weapon properly, you must feed power out of your body and through the weapon. This makes you much more powerful - especially for unarmed combat.