Astronomy has a long and proud history of contributions from amateurs. Amateurs are probably best known (at least to me) for discovering supernovae. I mentioned in another comment to this article that a friend of mine from grad school worked on a project called ASAS-SN, which uses a network of small, cheap telescopes to monitor the entire night sky every few days. Their stated goal was to use this network to discover supernovas (hence the SN in the name), but, of course, they have discovered a bunch of other interesting things as well. It was only a year or two ago that they discovered more supernovae than amateur astronomers. Prior to that, amateurs consistently discovered far more supernovae than professional astronomers. Interestingly, the most prolific discoverers were in Japan, because they could discover all the supernovae that went off over the Pacific Ocean that had already set for observers on the West Coast of the US.

Another field that involves lots of work from amateurs is microlensing. In a microlensing event, one star passes in front of another, and its gravity lenses the light from the star behind it. This produces a characteristic increase and decrease in the brightness of the background star over the course of a day or so. If the lensing star (the star in the middle) has a planet, this will distort the brightness curve. These sorts of observations are extremely time-sensitive, and its critical that data is collected during certain, very narrow bands of time. Weather or other observing priorities sometimes prevent professional astronomers from observing these events, so it's not uncommon that data in some of the crucial times are provided by amateurs.

And there are lots of other areas that amateurs have contributed enormously to! Asteroid discovery, monitoring variable stars (https://www.aavso.org/), and even exoplanet discovery! In many ways the term "amateur" does a disservice to amateur astronomers because their setups can be quite sophisticated --- the only thing "amateur" about them is that they don't get paid for all the great work they do!