Firstly, getting into a group and, secondly, the research you are able to do once you are part of the group.

In terms of the former, overt participant observation may give you access to groups who could not be studied covertly because the researcher does not have the same characteristics as the group.

In terms of the latter idea, if the researcher is able to join a group with their knowledge and permission it means they will have access to all levels of the group - something that's important if the group has a clear hierarchical structure, for example.