Mike Rapoport wrote:

Why? The added air has the same density of the outside air.

Good question. So I dug my books out. It turns out the internal bags (called
ballonets) are used for trimming and to allow the helium to expand and contract
without changing the shape of the balloon. They don't affect the buoyancy of
the craft.

George Patterson
A diplomat is a person who can tell you to go to hell in such a way that
you look forward to the trip.