What's the advantage of this versus Skype?
Not sure what "this" refers to. In my case, the big advantage is that SIP ATAs work without having a PC running. My mother-in-law has never used a computer. You connect a real phone (corded or cordless) with a real ringer to the ATA.
(SIP phone software is available too - eg Ekiga on Linux, SJPhone for Windows.)
Also, SIP is an open standard whereas Skype is closed and proprietary.
Possible minus point of the SIP ATA approach is that it doesn't provide the "online", "offline" etc info that Skype can.