For the student, a good internship provides some practical experience, perhaps future references, and establishes connections outside of school. A good internship also means that you will be exposed to substantive information to write a good research paper. I have found that the best internships are in public sector or non-profit agencies. The organizations in these sectors tend to provide students with a wider range of experiences than do private sector firms. I have had interns with law firms, and the results have been disappointing.
Law firms tend to have expectations higher than most undergraduate students can fulfill.
That is, law firms may expect you to be able to do the work of a law student. This can hurt you when it comes time for the internship site evaluation. In addition, law firms may simply use a student to fill out its clerical staff while never exposing the student to the legal profession. This makes it difficult to write a research paper. Internships with federal agencies can be problematic.