This comes from an LT application notes, but I found the topology in other circuits too.

The rest of the schematic isn't really relevant, this is the section I'm interested to:

It seems to me some kind of bidirectional clamp. The -15+15 resistors bias the zener with a good current point and the bridge steers the output voltage from the circuit before.

I guess that this would clamp the output to 2.5V + 2 diodes drops, am I right? Why the whole bridge instead of two zeners ?

Could it be that being the zener biased the operating point is stable so it wouldn't affect the signal (due to leakage or knee) before the clamp?