Mini vent attached to toilet base, why?

[KevinWeav2003]

Why does the base of our toilet have a mini vent attached? We have two toilets and this is the only one that does. We also have a septic tank if that matters, and of course a main drain vent outside. Thansk

 

[breplum]

Never seen anything like that in USA.
Likely, the trap of that fixture is very sensitive to needing "air admittance" when it flushes.
FYI, those devices admit air and prevent odors from escaping.

 

[havasu]