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The built in tuner usually works acrossa a limited range, like 3:1 or 5:1 SWR at maximum. A dedicated tuner is usually much more versatile.
And can include various other goodies, like built in 1:9 balun for using wire antennas and so on.
That's my experience with built in tuners. But I mostly use my MFJ–945E and LDG AT-200ProII.
But in general, I rarely needed tuners when using my OCF-D.
All the bands I used it on it was under 2:1 SWR, and in my books that's perfectly fine.