Advances in understanding and technology, as well as the failure of some probes to perform their tasks, make it important to send probes time and again to the same planet.
We don’t know what we will find when we receive information from a distant probe.
Every probe sent to Jupiter, for example, raises more questions than existed before. Voyager 1 discovered ten more moons orbiting Jupiter than we had seen from Earth.
To find out more about those satellites, further probes are necessary.
The first probes were only able to fly by planets, catching a mere glimpse of them.
When scientists figured out how to intentionally put a probe in orbit around another planet, we were able to see entire planets.
Nevertheless, there is only so much any one probe can accomplish in a mission.
Successive missions are needed to carry out further research. And consider a probe sent to the Sun.
If it passes Venus and Mercury on its way, we don’t want to miss an opportunity to receive new information by not turning it on as it goes by, even if other probes have been there before.