If the Spark contains the memories of Transformers then what does the head contain? Is it some sort of databank? For all we know Megatron's spark is still intact seeing as his head was ripped away. It begs the question if Megaton's spark is in his head like Frenzy. In which case, why did he die in TF1?
If you're Megatron, appearently your head contains what should be in the spark. Of course, Megs is evil and has no damn soul. So all his memories and evilness reside in his processor instead of the spark.
Optimus told Cade that the spark was their memories, etc., and Cade likened it to the soul. When Optimus was fighting Galvatron Optimus said he had no soul which means no spark. I assumed what was going on was the humans built Galvatron, gave him a power source, and Megatron just took it over by programming it how he wanted it. I think Megatron is evil and deranged enough to have a backup program stored in a databank in his head/brain and definitely used the humans idiocy to come back to life. The humans basically gave Megatron a new and fully functional body. Dumbasses.
That. Though I like to think that being resurrected by the Allspark has made Megatron into a Digital Dracula like character. As long as there is a body, he can be reborn.
That and all the sensory equipment they need, such as auditory systems, optics, olfactory nodes, radar, sonar, energon-scanners, communications gear, etc.
The brain. Y'know, as heads are want to do. Thoughts, cognitive functions. They say the spark contains memories, not that it is the sole receptacle for them. And Optimus doesn't mention thoughts or anything else, just memories. Huh, I wonder if that's a weird unintentional reference to the "backup memory" Brainstorm said all Transformers have in their chests in Rebirth.
Simple, Memories and personality are in the brain but some "believe" there is something that will allow them to survive beyond death. Remember prime's not an allknowing god,but a person in deep suffering. Men often gravitate to spiritual beliefs in such situations