It's a for profit company, but they still not only give away the source code, but offer a great deal of hosted functionality for free.

So contributing helps the community whether your contribution also helps the company or not.

Seems kind of like asking "Why would you give money to the Red Cross? They have employees that get paid a salary for running it."

If it's a good cause and you benefit from it, I certainly wouldn't avoid supporting it simply because someone else might profit indirectly from my contribution.