Sociologists often use observations as a research method. A non-participant observation is one where the researcher chooses not to play any part in what is being observed.
An advantage of this is that it allows for a more objective view of what is occurring. There is a danger that participant observers can "go native" and take on the values of those they are observing, because they share their experiences. However, you can argue that sharing experiences is more likely to offer verstehen than simply watching from the outside.
A disadvantage of most non-participant observation is the Hawthorne Effect - people are likely to change their behaviour because they are aware that they are being observed. This can be overcome with a covert observation (such as using CCTV) although that raises a number of ethical concerns.