I'm interested in real experience and personal answers not just those standard worn out "benefits of contributing to open source" that we all memorized by heart by now.
I've met many people who contribute to open source project and almost never did this topic come up: why did they contribute to this open source project. So, if you've contributed to an open source project before in any way, can you please pause for a second and try to remember what really made you decide to contribute to that particular project.
Was it a random decision, was it because you were bored, was it because the company you worked for was already using it and you contributed as part of your job, was it because the project was too big you wanted to get contracts, or the project was too small you wanted to build it, or because your prof or co-worker asked you to help with his open source project, or.....
To substantiate your reasons, please mention the project name and rate your involvement (heavy, occasional, light, once).