Then there’s optimisation. A circuit designed for one job is will do that job better than a multi-tasker. Smartphones have been compared to Swiss Army knives, which is fair, but you wouldn’t use one to cut pizzas. The Hi-Fi Plus module can’t take pictures or play games, but it sure knows how to handle music properly.
Finally, isolation. There’s a lot happening on a phone’s circuit board. Even without you cluttering it up with your food pictures, it's constantly doing things with Wi-Fi, Bluetooth, the display and so on - and that can cause electrical interference. It’s better to have an entirely separate circuit: imagine doing your homework at the dinner table, versus having your own desk with nobody to point at the spinach in your teeth.