There are instances where it may be difficult to predict file contents: NAAIRS offers the references but these need to be interpreted and sometimes the only way to do that is to access the original documents.
The code PWD (Public Works Department) doesn't sound promising for family history information, yet records concerning alterations to a bridge included a letter giving the ancestor's year of arrival in South Africa, previously unknown. This detail made it feasible to search passenger registers for that date parameter.
However, frequently the index reference may be all you need to establish what an individual was up to at a particular point in his South African career. A memorandum filed under CSO (Colonial Secretary's Office) could reveal that on a certain date the ancestor applied for an appointment in the civil service. In such a case, the file contents could add little to the index reference: it might be better to consult published civil service lists for the appropriate period.
It really depends on how lucky you are with finding references to your ancestor/s on NAAIRS. Where only one reference occurs it hardly matters what type of file it may lead to: at that stage it's a matter of leaving no stone unturned.