What are the uses of accelerometers for a robot?
What does the sensor’s output can tell me?
Thanks 
The sensor tells you the rate of acceleration (or deceleration) in a given direction. It has a large number of potential uses, for example:
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Bump detection. In autonomous mode, a sudden jolt can indicate another robot hitting you or running into an obstacle. If you dumped into the low goal during autonomous this year, you could use an accelerometer to tell when you hit the wall, and use that info to stop driving forward and start dumping.
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Wheel slip detection. While it’s not as accurate as some other solutions, it can help tell you if you’re wheels are slipping - if you increase power to the wheels and don’t see an acceleration, they’re slipping.
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Gravity detection. An accelerometer can detect the constant pull of gravity, which can be useful in some cases. For example, by detecting the direction gravity is pulling, a robot in Rebound Rumble could automatically tell if the bridge was balanced, and which way it had to drive in order to balance it.
Just some examples of uses… you’re team would have to figure out if it’s useful for your application, or if there’s another mechanism you could use that might be better suited to what you want.
How can the accelerometer tell the direction of gravity?
All objects, the accelerometer included, experience a constant downwards acceleration of 1G due to gravity. Most accelerometers measure acceleration in multiple axes. By measuring the acceleration caused by gravity in each axis, and performing some trig, you can figure out the orientation of the accelerometer.