PIO files (Archives of the Principal Immigration Officer, Cape Town) contain passengers' declaration forms for the period 1904-1967.
These records are extremely useful for family history: immigrants arriving at South African ports had to complete a form giving their name, age, birthplace, nationality, occupation, marital status, age and birthplace of spouse, reason for entering the country, port of embarkation and the name of the ship on which they arrived.
If a PIO file for your ancestor turns up on NAAIRS, it's a valuable find and should be accessed.
A good rule when searching NAAIRS is: don’t hit everything that moves on the index. A well-chosen list of references will keep costs down (if you’re delegating to a researcher), speed up the process and ensure that you make the most of available sources.