I teach in England rather than Scotland, where I assume you are based. I use group work most lessons, but then I do teach drama. However, I have also run training sessions for other staff at my school about the benefits of group work, and I do think that there are a great many.
For a start, it encourages independence. OFSTED are very big on independence - by which I mean independent of the teacher, rather than students being independent of each other. In a group, students can use each other, rather than relying on the teacher, which OFSTED like as it shows students learning by themselves, rather than being lectured.
Then there are the obvious social skills arguments - students have to learn to work with a variety of others - very few jobs involve working on your own with absolutely no communication with anyone, and so it is vital that they learn about cooperative working.
Personally, I find that students enjoy it. Again, that might be because of the subject I teach. I start off pretty strict in Year 7 - never letting students choose their own groups and by Year 9, they are very good at working even with their friends, and staying focused.
Of course, the key is organisation, and whilst OFSTED want (in fact, in the new framework, they insist on it) to see students apparently working independently of the teacher, whilst the teacher hovers and observes but doesn't need to intervene, it takes a lot of effective planning, training (for want of a better word) and developing students' skills to get it right. In my own classroom, I use a variety of methods for organising groups - anything from completely random, chosen spontaneously, to carefully planned groups - sometimes by ability, sometimes deliberately mixed ability. Given that I teach an option subject, at KS4 groups are sometimes by ability - I would hope that a group of top level students would come up with something pretty impressive, and then I can support the lower ability groups more (doesn't always work, of course) and sometimes mixed, as working with higher ability students can encourage the lower ability students to develop confidence, and working with lower ability students forces the higher ability kids to consider the clarity of their work (often in drama the top level kids can get a bit lofty and lose track of the basics
) Tasks have to be very carefully planned, and very carefully monitored. I constantly make notes, sometimes video group work (particularly at KS4 and KS5) and so I am always aware of any inconsistencies in effort.
Group work can also work for things like evaluations and self-assessment. I do a lot of putting a group of kids in a room with a video camera and getting them to record a discussion - often they will come up with more pertinent points than if they were to write things down as individuals.
No matter how the groups are organised, IME, marking and assessment is always done on an individual basis. I would never give one grade for a whole group .
That's very long! Hope it helps.