Whilst, the bottom line of Wifi Data offloading is that the user doesnt have to really pay for the radio resources, as Wifi is Unlicensed spectrum.
On the other hand it helps the operator in saving the demand for adding more radio resource elements (and ofcourse maintaining them) as his subscriber base increases. Licensed radio resource is a premium.
One more point is Wifi is still the #1 choice for internet access and I believe bulk of the Internet data is generated by Stationary hosts. By deploying Wifi offload, the mobile operators are attracting the Wifi users(Wifi being traditionally connected to DSL/Cable ISPs) and creating a huge revenue potential for themselves.
There are different offload technologies such as EPC offload, which means the operator will deploy a gateway to bypass the SGSN/SGWs and selectively route directly to the Internet.(Gateway having SGSN functionalities). It wasnt such a big success, however.
Wifi offload itself has different implementations from different vendors. Such as, offloading both dataplane and control plane to Wifi, offloading only dataplane to Wifi and still keep Control over Cellular, as Control plane constitutes important but lightweight(compared to data) characterestics.
3GPP 23.402 specifies eTTG, that terminates the SWu interface from the WAC and connects to PGW at the EPC over S2b interface. It also specified ePDG which doesnt connect to the PGW and instead directly talks to the internet gw(SGi) thus offloading the EPC.
Hope this answers,follow up questions..most welcome!