To Inject or Not To Inject UserManager?

gistfile1.cs

// The Identity 2.0 example project has a Constructor Override:

public AccountController()
{
}

// When would we do THIS?
public AccountController(ApplicationUserManager userManager)
{
    UserManager = userManager;
}

// When THIS exists?

private ApplicationUserManager _userManager;
public ApplicationUserManager UserManager
{
    get
    {
        return _userManager ?? HttpContext.GetOwinContext().GetUserManager<ApplicationUserManager>();
    }
    private set
    {
        _userManager = value;
    }
}

// Is there strong reason for or against injecting the UserManager?