The opposing console makes their console of choice better.
If Sony does something good, MS will try to one-up them.
If MS does something good, Sony will try to one-up them.
[spoiler]Gamesphere still reigns supreme though.[/spoiler]
That is all.
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A common practice in the world of business. It's called competition. Without competition many of our favorite brands would never be pushed to innovate with their products or incentivize because they would be aware that they are the only option.