The domain specialists know what their customers would like and are thus fantastic at evaluating proposals, but they don't know the range of possible solutions

The domain specialists know what their customers would like and are thus fantastic at evaluating proposals, but they don't know the range of possible solutions nearly as well as the people doing the implementation. Someone technical can propose something that the design side never considered because they didn't know it was even possible. Having people from the technical side in there early also means helps to spread understanding of why something is designed as it is.