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Why is that? What do they want to coroborate?

For planning observations primarily, amateurs can give them a broad idea of what is happening in the atmosphere as JUNO pass perijove on May 24 (closest approach). I believe they are planning observations using the VLT in Chile, but they have a limited time window that they can observe Jupiter.
Presumably, amateurs can help fill in gaps. From this side of the world I know Anthony Wesley (Australia) and Chris Go (Philippines) make high resolution images of Jupiter perhaps 2-3x inferior to Hubble. These guys are both amateur astronomers.

There is quite a lot of pro-am cooperation with astronomy.