Whether or not you need a cover letter has nothing to do with whether you've been referred.
If an office/company requires/desires it, write one! If they don't, no need…regardless of referral.
Generally, I view cover letters as table stakes items. As in, you need a solid one, but they're looked at quite briefly (if at all). That's why I developed a “mass production” technique for them!
My advice: Always have a cover letter. The only downside is time. Upside is: An offer