To catch browns.
I think it’s because they have a greater predator instinct.
I agree with ducksterman.
I do not like the way that brown was being handled. That fish needed to be released quick! Not hoisted by the fish it was trying to swallow. Too much stress on the brown.
Many yrs ago I was fishing at a lake near San Diego and there was a commotion. Someone had netted a big bass with another big bass in it’s mouth. Predatory fish don’t always use good judgement.
The bigger a brown trout gets, the food size becomes less of a issue.
Doug
Dear Gramps,
For the same reason fat people eat at buffets. When you at the top of the food chain you tend to want to stay there. 
Best Wishes,
Avalon 
Ever see the streamers that get chucked at bull trout?
Marty
I have…on the Metolius. I walked by the Dolly Hole and a guy was in from Portland throwing something big with a 7 wt. to imitate the rainbows the bull trout were chasing around.
I got a chance to look at his fly wallet and some of the streamers were 8 to 10 in. long…and…he caught and released a 5 pounder while I was standing there watching and speaking with him, so I know the pattern works.
Cheers
MontanaMoose
Heck yeah, I caught my first bull in October on the Metolius, down a little from the 99 bridge, on a 7" zonker.
And I’ve taken a few nice browns from time to time around here on 4-5" streamers.
Marty
One of the biggest browns I ever saw taken from the Battenkill was taken by a local kid using a blue Rapala that must have been 6 or 7 inches long.
AgMD
The annual bull trout fishing is fast approaching, and im tying 6 to 10 inch long streamers, on 2/0 and 4/0 hooks, with as much weight as possible.