Not really less latency so much as the signalling can be run at much higher speed and/or much lower power without having to worry about stuff like the optional

Not really less latency so much as the signalling can be run at much higher speed and/or much lower power without having to worry about stuff like the optional termination, the electrical properties of the socket->dimm connection itself, and much shorter track lengths. It also allows you to have as wide a bus as you want, limited by cost of packaging and die sizes on the package, instead of dimm size and board layout, which naturally has a much higher trace pitch than the package. The higher speed "may" cause lower latency, but it's likely secondary at best.