Not from perspective of design, but from open source software development.

1. Positive PR

2. External targeted contributions. For example, most people submitting patches to open source software are facing some issue as a user. If they are experiencing the issue others may also be experiencing similar problems. This is a much better way of driving development compared to doing it post expensive market research.

3. Marketing

4. Recruiting vehicle

5. Increasing the size of markets. For example Google is trying to bring more people online with Android to increase the size of search market.

6. Commoditizing complements http://www.joelonsoftware.com/articles/StrategyLetterV.html

7. Reducing the cost of training new employees

8. Increasing their influence in which direction to take the industry. For example setting standards for server hardware with Open Compute Project.

9. Free testers

10. Improving employee morale

Mostly free beta tester and patches BUT also increase a LOT your chances to hire.

Prospective employee's use public showcasing as a meter stick for who's who.

GitHub projects notably exemplify this -- not only is it a painless/cheap indicator of an employee's skillset, it also works for companies too.

On top of that, it fosters communication between teams in the industry and (in some cases) serves as a recruitment channel. Those that contribute in a meaningful manner to your open-source projects must meet certain standards and expectations, and finding such people is difficult. In this case they come to you.

I see this as a recruiting tool. Designers want to work for a company that "gets" designers' needs.

The large tech companies benefit from a thriving tech ecosystem: companies using these tools are much more likely to become partners or acquisition targets than they are to become direct zero-sum competitors.

It moves the industry forward? Also having the empathy to recognize that if you've built your success on top of free tools then it bodes well to contribute back to that same community that enabled your success. Think of it like an Alumni network.

Those answers are just awesome and exactly reflect the way how modern software business works. Please - big IT corporations - breath, understand, embrace and internalize what's written here. It should be part of every company strategy!!

It often attracts talented engineers; tech companies are always after talented engineers.

PR, influence, beta testing, 3rd party devs patching bugs for you for free ...

They get to spend less on training, making employees more commoditized.