The Warriors have 15 contracts presently.  That's their limit.  Barring trades or an unforeseen cut of a guy with a contract, the non-roster players aren't going to make the team.  I believe Hudson and Croshere signed one year guaranteed deals at the min that are likely guaranteed for the season.  Ellis also has a min cost contract for a year as does Azubuike (his has a second year player option so it's essentially a two year guarantee) but neither of them are going to get cut.  Everyone else has a bigger tag attached and are unlikely to get cut unless one of the nonrosters shows he's SIGNIFICANTLY better than one of low dollar guaranteed contracts.

Perhaps Nellie is evaluating just in case a move is in the works, perhaps there are potential moves in the works, but my guess is that Brown, Powell and Burke are practice fodder to make sure there's enough fresh bodies on the floor.  They never had jobs to lose and my guess is that they must know that they really don't have much of an opportunity to win a job either.  They need the guys to run some of the drills perhaps and there's guys willing to go to an NBA camp accordingly.  

Why did he play them?  Hard to tell.  Motivation to the kids?  There are other coaches who scout to grab cast offs who might be a better fit elsewhere. Perhaps there's some sort of promise to the guys who don't have a snowball's chance to make the roster that if they practiced hard, helped out to make camp run smoothly that they'd see enough floor time in a preseason game or two to potentially impress another GM either in the NBA or some other pro league.  If that's the case, game one would be the time to do it as any time closer to the launch of the season would be more disruptive to establishing a regular rotation for the regular 82.