what is the difference between use dbset and mappers in EF7

I started to work with .net core and Entityframework 7 in Onion Architecture! i readed this tutorial and i think its best case for learning following subject. but one part of this tutorial made a big question inside my brain. like what you see at this linked page; at Data Layer we have some classes which are our model!!

public class User : BaseEntity
{
    public string UserName { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }
    public virtual UserProfile UserProfile { get; set; }
}

public class UserProfile : BaseEntity
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Address { get; set; }
    public virtual User User { get; set; }
}

and some class which are mapping above models like this !!

public class UserProfileMap
{
    public UserProfileMap(EntityTypeBuilder<UserProfile> entityBuilder)
    {
        entityBuilder.HasKey(t => t.Id);
        entityBuilder.Property(t => t.FirstName).IsRequired();
        entityBuilder.Property(t => t.LastName).IsRequired();
        entityBuilder.Property(t => t.Address);
    }
}

public class UserMap
{
    public UserMap(EntityTypeBuilder<User> entityBuilder)
    {
        entityBuilder.HasKey(t => t.Id);
        entityBuilder.Property(t => t.Email).IsRequired();
        entityBuilder.Property(t => t.Password).IsRequired();
        entityBuilder.Property(t => t.Email).IsRequired();
        entityBuilder.HasOne(t => t.UserProfile).WithOne(u => u.User).HasForeignKey<UserProfile>(x => x.Id);
    }
}

and it use this mapper classes in OnModelCreating method of DbContext for creating following models as table, in database:

public class ApplicationContext : DbContext
{
    public ApplicationContext(DbContextOptions<ApplicationContext> options) : base(options)
    {
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        new UserMap(modelBuilder.Entity<User>());
        new UserProfileMap(modelBuilder.Entity<UserProfile>());
    }
}

My big question is this: we can use DbSet<> for each entities inside the db context and avoiding writing mapper and instantiating them in OnModelCreating method of dbcontext. why this tutorial didnt use dbset ?. why we have to create mappers!