Why would we want to use primary or secondary sources as evidence to support a thesis?

Primary Sources are directly from the person who wrote it or who witnessed the event therefor making it more trustworthy or better to use than a secondary source 

We want to use primary and secondary sources as evidence to support a thesis because using primary and secondary sources are more helpful and also gives you real evidence from real events that happened in life or/that came from a real person.

Political forces often influence public opinion.

Is that all any more to the question