The Last Day
The latest episode, 'The Last Day,' included an intriguing scene where Daisy headed out to confront an unknown threat. Whatever this threat was, it led the Government to evacuate an entire city. Daisy considered it so dangerous that, even knowing the likely aftermath, she chose to face it head-on. Given the involvement of gravitonium, it's possible that this threat was actually Dr. Franklin Hall, who was trapped inside a chunk of gravitonium back in season 1. Agents of S.H.I.E.L.D. repeatedly teased that Hall was still alive in there, and his emergence could easily be a global threat.
There are two important points to take from this. The first is that Daisy chose this. She knew the consequences if she acted; she was well aware that her actions that day would destroy the world. It's likely that she was dealing with a no-win scenario; that she believed the world would end without her intervention, and that this way humanity still had a chance.
The second point, however, is that Daisy had interacted with Robin. She believed it was possible to save humanity, albeit difficult. She chose this knowing her actions would set up a chain of events that would save the world.
Exploring the Time Loop
Ultimately, everything hangs on Robin. 'The Last Day' explained the show's interpretation of time-travel. Robin has used her powers to create a time loop, in which the present and the future are linked together. It's important to note that Robin is no impartial observer. She is actively using her powers to reshape history, to find a way to save the Earth.
As such, it's no wonder Robin's mind is fractured. She isn't just experienced one reality; she's experienced countless. No doubt she was the one who found the Lighthouse, which became humanity's last refuge. She will also have been the one to guide Enoch to the Monolith, and it was her advice that led the True Believers to build the "isochronous cyclotron," the tech they used to control the Monolith from the future. Robin created this time loop in order to save the world.
Daisy destroyed the world knowing she was creating a timeline in which Robin would strive to rewrite history. She chose to use her powers, to crack the Earth like an eggshell, because she believed Robin would find a way to change it. It's possible the loop has run thousands of times, with each subtle variation bringing Robin closer to her goal. In one flashback, Fitz - who survived the Earth's destruction, along with Simmons - noted that the team had tried to rewrite history "thousands of times before." His studies had made him aware of the time loop, and of the recurring element to it. Fortunately, it seems that this time Robin has had the vision she most longed for. This, ultimately, is the loop in which she has finally seen a way to save the Earth.
Little by little, Agents of S.H.I.E.L.D. season 5 has revealed how the Earth was destroyed. The team have a way home now, and it's up to them to travel back and see if they can't just avert this future. The time loop will have played out countless times before, whether in reality or in Robin's mind. Now, guided by the prophecies of the Seer, it's time for S.H.I.E.L.D. to do what they do best - and save the world.