Each person brings a perspective. As I mentioned, and quoting once again professor Cem Kaner,
Use “diverse half-measures”-- lots of different points of view, approaches, techniques, even if no one strategy is performed completely. – Cem Kaner, Black Box Software Testing
so each person has its own perspective, its own backpack of knowledge. Part of it may be overlapping, other not really.
Traditionaly, a developer has a greater backpack of knowledge in terms of the product, from a technical standpoint. Traditionaly, a tester has a greater backpack of knowledge in terms of testing.
Having developers and testers together, pairing, is for sure beneficial.
A conversation can start around the feature, from a funcional perspective, then go more to a technical side and back to the functional side.
In one of my teams, we have testers involved in mob-programming, being there to provide feedback and also do some guidance.