What is the purpose of the extension method CreatePerOwinContext in OWIN implementation by Microsoft

CreatePerOwinContext registers a static callback which your application will use to get back a new instance of a specified type.
This callback will be called once per request and will store the object/objects in OwinContext so that you will be able to use them throughout the application.

Let’s say you have defined your own implementation of IdentityDbContext:

public class ApplicationDatabaseContext : IdentityDbContext<MyApplicationUser, MyRole, Guid, MyUserLogin, MyUserRole, MyUserClaim>
{
    public ApplicationDatabaseContext() : base("<connection string>")
    {
    }

    public static ApplicationDatabaseContext Create()
    {
        return new ApplicationDatabaseContext();
    }

        protected override void OnModelCreating(System.Data.Entity.DbModelBuilder modelBuilder)
        {
        base.OnModelCreating(modelBuilder);

        // Customize your table creation here.

            #region USERS - INFOS

        modelBuilder.Entity<UserInfo>()
            .Property(p => p.FirstName)
            .HasColumnType("varchar")
            .HasMaxLength(70);

        modelBuilder.Entity<UserInfo>()
            .Property(p => p.LastName)
            .HasColumnType("varchar")
            .HasMaxLength(70);

        modelBuilder.Entity<UserInfo>()
            .Property(p => p.Address)
            .HasColumnType("varchar")
            .HasMaxLength(100);

        modelBuilder.Entity<UserInfo>()
            .Property(p => p.City)
            .HasColumnType("varchar")
            .HasMaxLength(100);

        modelBuilder.Entity<UserInfo>()
            .ToTable("UsersInfo");

        #endregion  
        }

        public DbSet<UserInfo> UsersInfo { get; set; }
}

and your implementation of UserManager:

public class ApplicationUserManager : UserManager<MyApplicationUser, Guid>
{
    public ApplicationUserManager(IUserStore<MyApplicationUser, Guid> store) : base(store)
        {
        }

        public static ApplicationUserManager Create(IdentityFactoryOptions<ApplicationUserManager> options, IOwinContext context)
        {
            var manager = new ApplicationUserManager(new MyUserStore(context.Get<ApplicationDatabaseContext>()));

            manager.UserValidator = new UserValidator<MyApplicationUser, Guid>(manager)
            {
                AllowOnlyAlphanumericUserNames = false,
                RequireUniqueEmail = true
            };

            manager.PasswordValidator = new PasswordValidator()
            {
                RequiredLength = 6,
                RequireNonLetterOrDigit = false,    
                // RequireDigit = true,
                RequireLowercase = false,
                RequireUppercase = false,
            };

            var dataProtectionProvider = options.DataProtectionProvider;

            if (dataProtectionProvider != null)
            {
                manager.UserTokenProvider = new DataProtectorTokenProvider<MyApplicationUser, Guid>(dataProtectionProvider.Create("PasswordReset"));
            }

            return (manager);
        }
}

In your Owin Startup you will register the callback:

// IAppBuilder app

app.CreatePerOwinContext<ApplicationDatabaseContext>(ApplicationDatabaseContext.Create);
app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);

which will call the static method:

public static ApplicationDatabaseContext Create()
{
    return new ApplicationDatabaseContext();
}

and

public static ApplicationUserManager Create(IdentityFactoryOptions<ApplicationUserManager> options, IOwinContext context)
{
    ...
}

Now you will be able to access your database context and user manager in a simple straightforward way:

ApplicationDatabaseContext dbContext = context.OwinContext.Get<ApplicationDatabaseContext>();
ApplicationUserManager userManager = context.OwinContext.GetUserManager<ApplicationUserManager>();

In your ApiController (if you’re using WebApi):

IAuthenticationManager authenticationManager = HttpContext.Current.GetOwinContext().Authentication;
ApplicationUserManager applicationUserManager = HttpContext.Current.GetOwinContext().GetUserManager<ApplicationUserManager>();