People have a tendency to drift away from each other, especially when they're doing completely different things.
Take design and development as an example. In the worst case, it feels like they're both on different planets.
And when that happens, prepare for nastiness:
😟 Unclear handovers
😟 Internal conflict
😟 Weird assumptions
😟 Unsatisfactory results
Unless they're brought together as a coherent unit, or product team, you'll find yourself having a hard time ironing out these issues.
A manager can't work miracles.
But there's a better way, and in my experience, all the clues seem to be pointing towards agile teams.