Jor-El saved his son by rocketing him off the planet Krypton just before it exploded. How and/or why did the original comic book Jor-El choose Earth as the target for his son's rocket?
The simplest answer which spans almost the entire canon of Superman iterations:
Jor-El chose Earth for its yellow sun (and the potential powers Kryptonians gained under a yellow sun) and in the earliest iterations the abilities Kal-El would possess as a result of Earth's lower gravity.
However this answer would not be fully explained until the 1960s. Earliest versions of the story don't ever explain anything more than Jor-El barely being able to get Kal-El off planet before its destruction.
First Superman Strip 1939 - Notice Jor-El (and all Kryptonians) has superhuman abilities on Krypton as a result of evolutionary development.
- The early comic strip versions only depict the powers Superman gained as a result of Earth's lower gravity. His increased speed and strength were the primary powers gained.
Other universe depictions of Superman had varying reasons in addition:
Early descriptions of Krypton sometimes had the Kryptonians having powers ON Krypton, so traveling to Earth would simply allow Kal-El to potentially lead or rule Earth if he so desired.
Earth was also chosen for its lower level of technology, hoping to offer Kal-El the most opportunity for survival as the last member of the Kryptonian species.
Some iterations indicated Jor-El had a macroscope or teleportation abilities allowing him to see/visit the Earth and have an understanding of the suitability of Earth over other potential sites.
Considering the reaction some universes had to the existence of Kryptonians, it was implied the Kryptonians were not always desirable neighbors, perhaps fear of their potential powers when they were off-world. This was noted particularly during the Man of Steel, Post Crisis era.