There are two reasons for the threshold. The first is protection: with too few responses, an individual's answers could stand out. Waiting until there are several keeps every contribution genuinely anonymous.
The second is accuracy. A single response tells you about one person's experience, not a culture. A handful starts to show a real pattern, and the picture gets sharper as more people take part.
If you look at a company and see that a score isn't available yet, it means fewer than five verified people have responded so far. As more do, the profile fills in.
A company page also shows a confidence level based on how many people have responded — so you always know how settled the picture is.