What Would Require Disassembly for Cleaning?

Generally speaking, buildup in the air ducts, air duct cover, and water tunnel after extended use warrants disassembly to clear the passages and improve air flow. 

The main reason for full disassembly is for access to the water tunnel (underneath the silicon insert).  Since the vapor passes through the water tunnel filtration after every draw, residue can get stuck after more use.  Refer to the "Disassembly" video to see how to access the water tunnel.

Otherwise, disassembly is helpful for keeping the glass clean and keeping the internal assembly dry after use. 

https://cloudious9.ladesk.com/654793-Disassembly-and-Assembly-Videos