Microsoft announced the Xbox One, the successor of the Xbox 360.
The Xbox One shall combine different entertainment aspects into one box.
Microsoft praised that the users only need the Xbox One to step into the next generation of entertainment.

However after the presentation of the Xbox One, Microsoft explained that the Xbox One won't be backward compatible to the Xbox360. That means, if the users want to play Xbox Live Arcade Games or Xbox 360 Retail games, the people shall buy a Xbox 360 beside the Xbox One.

This approach is completely opposed to the goal of the Xbox One (to have only one box for entertainment in the living room).
That's why Microsoft shall add Backward Compatibility for the Xbox One to play Xbox 360 Games.

Many Xbox 360 owners have an huge game library (Xbox Live Arcade + Xbox 360 Retail), which they want to use also on the Xbox One. Especially if you are collecting games or game series it would be perfect to play them on one console. Only to name some examples for game series which started in this generation and will continue in the next generation: Assassins Creed, Halo, Alan Wake, Mass Effect etc.

Furthermore Microsoft can launch a console with an huge library of available games. The customers don't have to decide between the Xbox 360 and the Xbox One, if the Xbox One could offer backward compatibility to the Xbox 360.
Thus new Xbox One customers can buy Xbox 360 games and increase the profit of Microsoft, the publisher and the developer, too.

Additionally it would be customer-friendly to add to possibility to play Xbox 360 games on the Xbox One. It's an advantage over the competitors.
One Box which has the ability to play the games which were released over the last 10 years.

To sum up, Microsoft shall add Backward Compatibility for the Xbox One, which has an advantage for Microsoft and the customers.