In your first comment you said, "why would I pay for [software] if it is provided for free?" And you're right, you don't. What you pay for is for someone to develop software/features that don't exist yet. Maybe the users of the phone you sell want some new feature. Then it's done and everyone's happy and even better you don't need to pay for the entire project to be built from scratch or pay ongoing license fees or whatever. That's how you monetize open source.