Maybe "trust" in a purely technical sense, but we live in a society. If a vendor has a part of their contract that would put them under penalty of fraud, customers don't usually worry whether they can independently verify it's true.

For instance, if you write in your agreements that data goes into this bit of open-source code and leaves it and goes nowhere else, and you list out some processes you take to ensure this is true, and the whole thing is an outright lie, that's really bad for you in ways that are clear and enforceable. It's also hard to hide from your own employees, and people come and go. You're unlikely to make that claim unless it's true.