Jim Crow laws were designed to prevent blacks from voting in the old south. Voting laws were only 1 type of Jim Crow Law. In general, Jim Crow Laws mandated the "Separate But Equal" status of blacks in the south. The laws ensured segregation, but not equality.
The reason they prevented blacks from voting was so that the Democrats could keep the power. Because if the blacks could vote, they would vote for the Republicans Jim crow laws were laws that enforced segregation. Its a legal way to prevent African Americans from voting. From Britannica.com
Jim Crow law, in U.S. history, any of the laws that enforced racial segregation in the South between the end of the formal Reconstruction period in 1877 and the beginning of a strong civil rights movement in the 1950s. Jim Crow was the name of a minstrel routine (actually Jump Jim Crow) performed beginning in 1828 by its author, Thomas Dartmouth (“Daddy”) Rice, and by many imitators, including actor Joseph Jefferson. The term came to be a derogatory epithet for blacks and a designation for their segregated life. From spartacus.schoolnet.co.uk
Jim Crow laws were tested in 1896 by Homer Plessey when convicted in Louisiana for riding in a white only railway car. Plessey took his case to the Supreme Court but the justices voted in favour of the Louisiana Court. William B. Brown established the legality of segregation as long as facilities were kept "separate but equal". Only one of the justices, John Harlan, disagreed with this decision. In the early 1950s the National Association for the Advancement of Coloured People concentrated on bringing an end to segregation on buses and trains. In 1952 segregation on inter-state railways was declared unconstitutional by the Supreme Court. This was followed in 1954 by a similar judgment concerning inter-state buses. However, states in the Deep South continued their own policy of transport segregation. This usually involved whites sitting in the front and blacks sitting nearest to the front had to give up their seats to any whites that were standing. African American people who disobeyed the state's transport segregation policies were arrested and fined. In 1956 African Americans, led by Martin Luther King and Rosa Parks, organised the successful Montgomery Bus Boycott.