Because they can brake of a piece of the planet an use it as samples and figure out what its made of or if its useful to us or can there be life on this planet because some times when a drill on a probe drills in a planet they can find water or other resoures.
aliens
yes
Huygens
Because Cinderella's fairy godmother tolled the probes to go to another planet first.
Mars.
Jupiter
Yes, probes have.
Currently, Mars
having fun
Probes have landed on Venus
As of 2014, humans have not set foot on the planet Mercury. To date, the only probes sent have been Mariner 10 which did a flyby, and the MESSENGER orbiter; both probes from NASA.
The blandest appearing planet based on visuals from space probes is likely to be Venus. Its thick cloud cover hides its surface details, giving it a uniform and featureless appearance when observed from space.