A lot of players play in positions early that's not their main position. Half back is a good position to learn how to play a game that's balanced between attack and defence. Half back at AFL level is a similar role to what a defensive wingman was many years ago. It might also be a position he can play and not be too stuffed to train hard mid week. He will play some midfield this year, certainly but developing players to be multipositional is really important and if he goes well it increases his chances of making the AFL team.
I would think he would play a fair bit of midfield next season and I would expect him to really come on then although for what he is I think his 3rd season will be when we see what he can really do. No doubt the club would love if he can nail down another position so he can rotate through there. I feel we will want to build a team which has plenty of successful rotations in it. We will want to be able to keep players fresh and run over the top of sides. Bigger picture here, player development is not just developing players to their strengths it's about improving their weaknesses and adding strings to their bow.
Wouldn't it be great if Stocker became a really good half back as well as a mid like Docherty is and to have both of them in the same side and able to rotate through the center and backline.