Why/Should we implement BaseColumns when using a Content Provider in Android?

I was browsing the source code for Google IOSched App and noticed the following code snippet as part of their Content Provider implementation:

public static class Blocks implements BlocksColumns, BaseColumns.

As far as I know BaseColumns is simply an interface to two constants: _COUNT and _ID.

I have two questions:

1. What are the advantages/disadvantages of the implementing BaseColumns as opposed to having a private field _ID in the class directly?

2. What is the role of the constant _COUNT?