The sample detector can be used to measure both CW signals and noise, thus it is useful for making Signal-to-Noise ratio measurements. However, it is harder to use than peak detector for CW signals, and slower to average for noise. To measure a CW signal, either the RBW must be significantly wider than a display point or you must carefully tune the analyzer to get the peak of the CW signal. To measure noise, you must use some form of averaging.
RF Detectors 560 & 5400 Series Operation & Maintenance Manual
RF Detectors 70/73/75 Series Instruction Sheet