Weapons are implements in the truest sense of the word. After you begin to gain abilities and improve in the open-hand form weapons allows you to test out your abilities and hone your skills. knowledge from the open-hand form speaks directly to the movements of all the weapons. Your open-hand form and weapon form will benefit jointly as you progress.
Why study weapons forms?
They are interesting, beautiful, and a really good time.
The weapons forms are all a lot shorter than the open-hand forms so if you finished an open hand form you are in for a real treat.
They include movements from the open-hand forms so your other forms will improve and these forms will not be completely foreign.
Think of a weapon as an implement. Its weight, length, and material are designed to teach you a specific ability.
There are four main weapons that all main styles share, the sword (Jian), broad sword (Dao), spear (Qiang) and long pole (Koon). Training in Tai Chi weapons can dramatically improve your tai chi.