First, Aricebo as a space observatory was a serendipitous accident. It was originally designed just to be an atmospheric observatory. The original idea and calculations for the dish were done in a masters thesis (maybe doctoral) and, it being the late 50s and early 60s, almost immediatly got funding just like anything else at the time with physics or science attached to it. Fast forward a few years. They build the thing and realize the student made a factor-of-10 error in his calculations, but in the right direction. Arecibo was 10x LARGER than it needed to be to do the atmospheric experiments it was designed for. Turns out, they could bounce radio waves off the moon and get a response. They could hit Mercury and hear something. They eventually could even hit Jupiter. (Side fun fact: the range was limited not by the sensitivity of the receiver, but by how far the earth rotated during the flight time of the radio waves. Anything past Jupiter and Arecibo rotated out of the receiving window before the radio burst could return.)
Second, Arecibo isn't just an active radio telescope as mentioned before; it's an active microwave experiment. To study the dynamics of atmospheric plasma, Arecibo could focus microwave transmissions at different altitudes, exciting small patches of air into a plasma. It would then immediately listen to study the cooling and reionization properties of the atmosphere.
Arecibo had a strong run, providing scientific insights for over 50 years. RIP, Arecibo. You maybe have been nestled in mountains down here on Earth, but you were still a Great Observatory to me.