The data logger normally consists of one or more internal sensors (more rarely we find them connected externally) and stores at specific intervals the measurements taken. This data can then be analyzed later, interfacing the data logger to a PC, tablet or smartphone.
The data logger does not force the physical presence of a person during the data collection, it allows a thorough analysis over time of the relevant measures. In short, we can have a large amount of data to analyze without a person being constantly engaged in the task of collecting data.
Data loggers are widely used in various fields, e.g. for measuring weather data or for storing the route tracked by the GPS.