The academic community largely gave up on the 'eight strangers in a room' focus groups as a useful and reliable technique some decades ago, but they still persists as the ‘go to’ method for a lot of commercial research. Why?
They are quick and convenient, relatively inexpensive and viewable by clients (the infotainment factor). The less research literate internal clients like them (they think focus groups are proper research). The rationale is often that discussion is generative; that people will bounce ideas off each other (although this doesn't happen very often).
On the downside, they often produce data that is paper thin. They are far too easy for a moderator to manipulate (sometimes under pressure from a client to produce an expected or desired outcome) and they are often misused as a quantitative substitute ("how many people in the group liked product A more than product B?"). Group dynamics are frequently a problem, providing disproportionate voice to the few dominant, extrovert characters. There is also a tendency for clients to latch on to the one or two things that they recall (or agreed with) from the one or two groups they viewed and regard this as 'the insight' .