This is a good question, and actually is not something that is very well understood. The "why" that you ask has a couple possible meanings here, and so I'll try to address both. By your question, you might mean "why is it useful to store visual memory more than audio?". This question is not easily answered by science because it can't easily be tested. We can't really design an experiment (yet, at least) that changes how someone or an animal stores memory and then look for an advantage or disadvantage compared to a person who stores it "normally." So in that sense of "why," the answer can't be approached yet. If by "why" you mean "what physical process causes this to happen," that is a question we can answer.

The short version is that there are two different pathways to your brain that record things differently. Nerves in your ears that sense sound send one signal, and nerves in your eyes that receive light send a separate signal. These signals don't merge before being wired into your brain. You can think of it like wiring - it is actually very much similar.

I am not sure what you mean by stored twice, but there are two possibilities. First, you might mean that we have two eyes, and so we look at things with both eyes and they send nearly identical signals to the brain, because both eyes are focused on it. On the other hand, each ear is focused primarily on one side of hearing. This is one aspect of this concept. Another might be that when you see something, your brain receives a pattern. When it is learned, that sometimes involves a part of your brain (called the hippocampus, it looks like a seahorse) replaying the memory of it. But, this is also true for things you have heard, though such a signal seems to be less strong. These pathways are currently under study in a lot of labs in the world (including some I have worked in/with) and the answers are not yet crystal clear. I think they will be much more so in the next 20-30 years, which means that if this interests you, it's definitely something that you can study in the future and maybe discover some new things of your own. There's a lot of opportunity for it.