Why would a researcher use a secondary source instead of a primary source when analyzing a historical event

Primary source is the firsthand experience and evidence of an event, where the any given information first appeared (initial report). The secondary source uses the first source in order to give its information (for example: document that analyzes primary source documents).
When analyzing historical event a researcher use a secondary source instead of a primary source because he uses the primary source as the imput information (facts, events, dates, names) in order to analyze them, connect them and explain them in his work.

Cherokee  removal, part of the trail of tears, refers to the  forced  relocation between 1836 and 1839 of thecherokee  nation from  their lands ingeorgia, south carolina, north carolina, tennessee, texas, and alabama to the indian territory (present day oklahoma)  in  the thenwestern united states, and the resultant deaths