Eli5 –> Speed of Gravity and GW170817 – Why was it a big deal anyway and won the Breakthrough of the Year award for 2017
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Grumblingduke has given a fantastic response relating to the speed. I will add a bit more about why GW170817 is a big deal and won the Breakthrough award.
As well as allowing us to measure the speed of gravitational waves, it allowed us to measure the distance to the galaxy that the collision occurred in (for more info on method, look into “standard sirens”) and by then looking at how fast the galaxy is moving away from us we can figure out the expansion rate of the universe. While we already had more accurate methods of measuring this, there are concerns about the other distance measures we use, and so having this method that is independent may be quite important.
Comparing the expected amplitude at that distance to the actual amplitude also allowed a test to a number of general relativity alternatives with extra dimensions, and ruled out the gravitational wave “leaking” into extra dimensions. Going back to the speed measurement again, the fact that the speed was the same ruled out several GR alternatives.
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