It is very impressive that they can control the return trajectory accurately enough for a tower catch, and to coordinate the arms on the tower (which are very slow and heavy), to grab the rocket without damaging it.
But this was already convincingly demonstrated by catching the booster three times.
It is very hard to return the second stage from the orbit due to its vastly greater kinetic energy. Protecting the rocket from the heating during the reentry is something that has never been done until SpaceX started to practice with the Starship.
But they have already demonstrated this feat before, with pinpoint landings into the ocean.
The significance of today's event is mostly in demonstrating that they have such a high confidence in this system that they were able to satisfy the safety requirements for obtaining a licence to fly the Starship return trajectory over the populated landmass, crossing Mexican territory on the way to the tower. This is the cherry on the cake. Now all the pieces are in place, and once the remaining wrinkles in the system are ironed out, we can expect the flights to become routine, and eventually, the reuse of Starship to become routine as well.
The other, somewhat more modest, but still important milestone was that they kept the Starship in orbit for a whole day, and after that they still had the fuel for a deorbit and the landing burns. It did not all boil off.
Edit: Here is an interview with Lars Blackmore, the guy in charge of Entry, Descent, and Landing (EDL) for Starship, where he talks about flying the descent over land for the first time: https://youtu.be/POrMxoZbXVI?t=390
But this was already convincingly demonstrated by catching the booster three times.
It is very hard to return the second stage from the orbit due to its vastly greater kinetic energy. Protecting the rocket from the heating during the reentry is something that has never been done until SpaceX started to practice with the Starship.
But they have already demonstrated this feat before, with pinpoint landings into the ocean.
The significance of today's event is mostly in demonstrating that they have such a high confidence in this system that they were able to satisfy the safety requirements for obtaining a licence to fly the Starship return trajectory over the populated landmass, crossing Mexican territory on the way to the tower. This is the cherry on the cake. Now all the pieces are in place, and once the remaining wrinkles in the system are ironed out, we can expect the flights to become routine, and eventually, the reuse of Starship to become routine as well.
The other, somewhat more modest, but still important milestone was that they kept the Starship in orbit for a whole day, and after that they still had the fuel for a deorbit and the landing burns. It did not all boil off.
Edit: Here is an interview with Lars Blackmore, the guy in charge of Entry, Descent, and Landing (EDL) for Starship, where he talks about flying the descent over land for the first time: https://youtu.be/POrMxoZbXVI?t=390