Does it usually happen even if all of the adviser's students have very different research interests?
These meetings can serve a variety of purposes, but many of them fall into the broad category of keeping the adviser and all the group members updated on what everyone in the group is doing. Below are some of the secondary benefits of a group meeting:
- saves the adviser time (compared to meeting with students individually)
- motivates the students to keep doing work, so they have something to report at the meeting
- fosters a sense of community with the group
- provides the students experience giving informal presentations
- often leads to the more senior students mentoring the junior students
To answer your follow-up question in the description, no, some faculty meet with each student individually; in fact these separate meetings are common, for example, in mathematics.
As one of these advisors who has both individual meetings with students as well as group meetings, I can attest that I use the group meetings more for skills development rather than just rehashing research-related issues.
It serves a number of purposes:
- Presentation of skills and tools that will be useful to the entire group (for example, in my group, that means programming tools, content management software like Git or Subversion), as well as things like "How do you write an abstract?"
- Presentation by group members of their own research work
- Presentation by group members of other people's work (often through a discussion of a paper in the literature, or a group of papers clustered around a common topic)
Now, my group happens to be bifurcated in purpose: I have people working in two very different application fields. I still have everybody present to the entire group for the simple reason that if they can't inform group members with whom they have methods and techniques in common about their research, how are they going to explain their work to anybody else?
Aside from Dan's answer, which is probably one of the main points, having the lab members present in group meetings also helps significantly in developing public speaking skills in a low-pressure environment. Many of the lab meetings that I've attended focused on the ability of the presenter, often a graduate student, to present an idea clearly, concisely, and persuasively. As an added bonus, at well-run meetings, there are often many questions, which helps the presenter learn to field (sometimes difficult) questions on-the-fly. These skills are pretty important in academia.
The main purpose is to waste some time. Probably the only thing that does not happen during these meetings is work. Unfortunately, lot of managers and professors think that we need to meet in order to know what everyone else is doing (seen from PhD position). We do meet each other in the coffee room, in the lab, while commuting, after hours, in the elevator and so on. We talk. We know. If we have a problem, we talk. Sorry for the rant, but I had to counterweight all the positivity in other answers.
While I generally agree with the other answers, I see three additional purposes of group meetings that I cannot accomplish easily in my individual meetings. While the meetings are weekly, I like to have lab members think about the upcoming month.
- Notifications about upcoming events: I might tell people about an upcoming conference deadline at our individual meetings, but I often will forget to tell someone.
- Management of shared resources: Getting everyone to briefly say what they are planning to do in the upcoming weeks often highlights that there is going to be high demand for a resource and lets us work out alternative arrangements
- Managing my (PIs) time: Maybe this is also a shared resource, but I try and let people know my teaching, marking, reviewing, writing, etc loads. I also try and get people to tell me what they want me to do over the next month. I think this helps people (both myself and lab members) adjust their schedules so that we can tackle tasks efficiently.