Why what SpaceX did is important ( short version )
On 2015–12–22 SpaceX successfully landed the first stage of a Falcon 9 rocket back at Cape Canaveral after a flight to deliver a payload to orbit. Pages and pages have been written about why it’s important but here’s the short version.
Lots of authorities on the subject said it could never be done. Lots of authorities on the subject said SpaceX was wasting time and resources trying. They did it anyway. Tasking themselves with the impossible and delivering is the most important thing SpaceX accomplished.