Traditionally--theologically and philosophically at least--god is supposed to be the perfect being. Being omniscient is part of being perfect.

There are all sorts of possible problems if god is not conceived of as omniscient. If god is not omniscient, then by definition there are things that god does not know. This means that god is fallible since he can make mistakes now due to his ignorance. He can misjudge people for example.

If god is not perfect, then even if there is a 'creator' what you are left with at best is just a very powerful, very smart being. That could just be some super-smart super-advanced alien for all we know.