After all, they must be using helium to inflate it, otherwise it would be too heavy
The 1400kg of mass is in the very thick (over 30cm), multi-layered walls, not the air or helium filling. Inflatable doesn't automatically mean "thin-walled and light."
and too much of a fire risk
Inflatable also doesn't automatically mean flammable. The inner layers are Nomex for fire resistance, followed by 3 separate sealing layers, rip-control layers, and then the outer Nextel ceramic fiber layers spaced with foam layers. The outer layers provide insulation and Whipple-type shielding, per the fifth figure here.
Here's the scheme of 24 to 36 layers used in the Bigelow Genesis modules and planned BA 330, which stay close to the Transhab arrangement.
And to answer your original question: it's an almost-empty module occupying a docking port on the station that could host future modules or visiting spacecraft. Two years is sufficient to gather on-orbit observations of materials performance not possible with the unmanned Genesis demonstrators.