Programmers of Android apps may need to link the software app to the phone hardware in someway or the other. There are primarily two ways of doing this:
1. To link to IMEI number
2. or to link to Android ID.
The following thread from stackoverflow.com explains both ways of doing the same:
http://stackoverflow.com/questions/1972381/how-to-get-the-devices-imei-esn-programmatically-in-android