Question

why our respiratory tract is moist?

Answer 1

The respiratory tract are kept moist with the help of mainly trachea which are lined by some specialised cells especially for that function. The trachea is lined by pseudostratified epithelial cells (mentioned above). They bear some glandular cells (mucosa or mucous glands).
The secretion of the mucus glands keeps the walls of the tube completely moist and traps dust particles in air. If the following tracheal components are not kept moist the latter might lead to emphysema and blockage of the lungs (as the dut particles are no longer getting trapped into it) ordinarily will destroy the lungs due to dust deposition in it.