Group interviews can tell you things that one-on-one sessions cannot. They allow users to build on each others responses and come up with ideas as a collection. While there are certainly advantages contextual inquiries, group interviews provide checks and balances to keep other interviewees in check—which makes it easier to sway away from the minutia and focus on what matters to the majority.
Projects often create focus groups because they are inexpensive and can be done on a short schedule, but some of the drawbacks include not hearing the whole picture. In a group setting, some users may not share their most intimate troubles to the interviewer in fear of embarrassment.
This isn’t something you want to try on your own. It requires the skills of an experienced moderator—and you want to do many sessions to gain perspective on your entire user base to bring objective feedback to your team.