Matched brand systems—like pairing a Smith helmet with Smith goggles, or Oakley with Oakley—are designed to work together seamlessly. The biggest advantage is eliminating that annoying "gaper gap" (the exposed strip of forehead between your helmet and goggles) and ensuring the ventilation channels line up properly, which helps prevent fogging.
The fit tends to be more comfortable too, since the helmet brim and goggle frame are shaped to complement each other. You won't have goggles pressing awkwardly on your nose or sitting at a weird angle.
The trade-off is that a matched system may not play as nicely with other goggle brands down the road. If you already own goggles you love, or you like to swap between pairs, a universal helmet gives you more flexibility. But if you're buying both at once and want the cleanest, most integrated setup, going matched is worth it—just try them on together first.