Answer:
To corroborate a primary source's claim about an event
Step-by-step explanation:
By checking different sources, you're actually making sure that you have different opinions and points of view about the same subject. Not only that helps with the reliability of the source but it also might reveal quite a few things about the same vent being studied. If 3 secondary sources differ from a primary source, for example, maybe there's something there that needs to be revealed or understood.