So, there are sort of two answers to this.
Answer number one is that this has already been done on earth, although not QUITE to the scale you're envisioning. Telescopes in general have better resolution and performance based on the diameter of the viewing aperture (either the recieving dish for radio telescopes, or the lens for an optical telescope).
The Arecibo Observatory is built in a sinkhole, rather than an impact crater, but in the 1960s it was cutting edge for radio telescopy. China has since built an even bigger one, although Arecibo still wins in my book because they have space cats!.
So, there are absolutely good reasons to build a Really Huge Dish in an impact crater.
Now, that said, those reasons have mostly been superseded by the introduction of high speed data transfer abilities allowing a technique called Aperture Syntheisis. Instead of buliding a dish 500m across, you can get the same effect by building two smaller dishes 500m apart from each other. If you're thinking of building a dish on Saturn in the first place, whatever result you're looking for is going to be met far more effectively by putting a constellation of smaller dishes in orbit AROUND Saturn. A dozen networked telescopes in geosynchronous orbit around Saturn would give you an effective dish size of almost 200,000 km, if I have my math right.