Commonly used to protect moving parts and to allow flex.
Offer protection from the elements, invariably dust protection.
If sealed at either end, it protects against fluids / oils ingress
Common places these are used are to protect cables – handbrake cables as an example.
Speaker cabling in car door when the doors open the bellows expands but maintains the protection against weather. Prevent wires from being trapped. Hides things also.
Bellows are used between two moving parts e.g. between train carriages
It protects a flexible joint
Typical material used is Natural / Neoprene or EPDM.