"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
Apple With Specific Plans for P.A. Semi
According to their source, Apple had already been an investor of the company and had previously been in negotiations with it for low-power PowerPC chips. Apple's switch to Intel, of course, ended these talks and eliminated a potentially large revenue stream from the company.
More recently, Apple is said to have had a new chip design in mind and wanted the P.A. Semi team involved. According to EETimes, P.A. Semi had essentially run out of funding and "the only way to get the project funded was for Apple to pay off the other investors and bring P.A. Semi in-house."
As a result, the $280 million acquisition was to bring their talent in for this new project rather than any specific technologies P.A. Semi had already developed. This explanation is consistent with early statements from P.A. Semi to their existing customers that the company was bought for "intellectual property and engineering talent".