Spider-Man's Aunt May is one resilient old lady - she's been targeted by some of Peter Parker's most fearsome foes and held her own. When the writers decided to kill her off, it marked an important transition in Peter Parker's life, as he moved on to adulthood and a marriage with Mary Jane Watson. There was only so much drama to be milked out of the woman, after all.
Apparently that well wasn't dry after all, as just a few years after her death it was revealed that the May Parker who died in the hospital was actually a "genetically altered actress" who replaced the real May while she was being held captive by villains. Why? Because people are dumb. May being brought back to life enabled Peter to make the deal with the devil that would eradicate his marriage from continuity, so that's just the cherry atop the poop sundae.
