I agree with most of what
@Mikerb says above.
Getting through tree gaps, tick.
Shoulder width, tick.
Wider bars improve steering control, tick. This was the biggest reason I went wider. Over the years, I went in steps from a starting point on my XC bike of 660, to 680, then 720, then 740, then 780.
My bike has 65.5 deg head angle and I have 780mm wide bars with a 20mm rise, flat stem. The bar end plugs add max 5mm total.
I tried wider bars, but I kept snagging on tree gaps. On popular trails you could see from the gouges in the tree bark that it was a common event for many riders.
My wingspan (fingertip to fingertip) is the same as my height 6'1".
If you go wider, your centre of body mass will move forward, this may alter your ride in a way that you might like, or not. Shortening your stem length can restore this body position.
Note: My grips don't add 10mm per side (yes, I know he said
most grips add 10mm per side. It's the "most" bit I disagree with)