Considering that Donovan Mitchell and Kemba Walker are average 3-point shooters at best, it makes sense to have someone out there whose sole role is to drain open 3s. And Harris shot over 47% from 3 last year, which is... nuts.

Also, Harris hasn't started every game or anything. Pop has been trying a variety of starters. I imagine when the real tournament starts, it'll be Walker and Mitchell as the backcourt, Turner at center, and the last two are up for grabs. But if he wheels out, say, Jaylen Brown and Harrison Barnes as his other two starters, then there isn't a single guy on the floor who's known for being a "good 3-point shooter." I'd strongly consider starting either Middleton or Harris in one of those spots for exactly that reason, and then either Brown, Tatum, or Barnes as the fifth starter.

(You know who I'd pick, but I'm obviously biased, lol-- admittedly Tatum hasn't played very consistently in these prelim games, so wouldn't surprise me if he's more of a first-off-the-bench guy.)