Ideally for Armstrong he'd be playing central midfield for Everton, but as that's not likely right now the option for being involved in another position or as a substitute in Premier League games, involved in training, and just being a valued member of the team is much better than playing in any position for Preston. It'll also be much better for Everton in the long term.
Armstrong staying at Preston would have been ideal for Preston, but I don't see why that should be important to us. Good Everton players should obviously be playing for Everton.