The purpose of pull taxes in the southern states was to

Restrict the ability of African Americans to obtain voting rights.
The black community experienced much poverty because of prejudice against  them in the economic system of the country, so poll taxes could keep them from going to the polls to vote.

Blacks also typically had less access to education than whites even after slavery was ended; thus their literacy rates were lower.  So literacy tests were also used to restrict blacks from voting.

Southern states also utilized "grandfather clauses," which were exemptions to things like poll taxes and literacy tests, granted to those whose forefathers ("grandfathers") had full voting rights prior to the Civil War.  So if there were poor or illiterate whites, they could vote freely while blacks (whose ancestors had been slaves) were subjected to the laws restricting their voting ability.  
These sorts of restrictions against black voters prompted much of the activism of the civil rights movement that began in the middle of the 20th century.