While technical competence is important, hires can go bad for a number of reasons, all of which companies want to avoid. Companies want to avoid hires who are:
- lying on their resume
- a fraud
- arrogant, bigoted or otherwise hard to work with
- a bad fit for company culture
Someone who knows you should be able to verify that you are none of these things, even if they work in a different area of expertise than you and/or have no idea of your technical competence. That's useful information to the company, and already makes you a better hire than many people where the only thing they know about them is their resume.