Appium is a mobile automation testing tool.
Benefits of Appium :
Some advantages are listed here
a.Supports both Android and IOS platform
b.Supports all kind of Application (Native, Hybrid and Mobile Web)
c.Supports most of API levels for Android
d. It has support for multiple languages to create the test
scripts, though Java is most used one.
e.Very activity community, if you need a support.
f.Both Selendroid and Robotium are primarily for Android Apps.
When you create you Automation script, you also need to think about how do you run your scripts on real devices. I would suggest you to evaluate some of the device cloud providers and see which fits best. I have used pcloudy. It allows seamless run of Appium scripts on multiple devices.