|The show needed more Mike.|
So, they wanted to have a bunch of stuff from the past, and thus, we have a time-space breach to a Fear Dimension. Which is tonally a bit out there for the season and the series. And so we sort of get Lash, and we sort of get Hive, and we sort of get a bunch of other stuff, but mostly we get Mike Peterson/Deathlok.
Backing up, though. Once the Fear Dimension is set up, most of the episode is spent with everyone else learning what Coulson and Yoyo already knew - Coulson is dying. It turns out it’s a direct result of his deal with Ghost Rider, essentially negating the process that brought him back to life, and quickly redeadening him. People react pretty much like you’d expect, given how their characters and relationship with Coulson have been set up.
But the big, emotional set piece of the episode is about Coulson’s biggest fear - the idea that he never came back to life. The idea that the entire run of Agents of SHIELD is an elaborate fantasy concocted by his dying brain as SHIELD tries to bring him back to life. Or, as How Did This Get Made fans know it, a classic Jacob’s Ladder Scenario. It’s obviously not true, but they do the work and make it just convincing enough that you believe Coulson could be convinced by it as long as he is. Not super long, but it doesn’t need to be super long. This idea is put forward by the mental ghost of Mike Peterson, who then returns for real as Deathlok, before going away again for no convincingly good reason beyond contractual obligations for J. August Richards.
Oh, and Fitz and Simmons got married, which is nice and fine.