It is unknown what Dexter was attempting to do with it once it was completed, but after he completed it, he released an Inter-Dimensional Monster that ran around his laboratory, destroying everything.
When the popular kids began making fun of Dexter and Mordecai, Mordecai reprimanded them, saying Dexter was way cooler than them for having an Inter-Dimensional Doorway. Showing that Mordecai knows about Dexter's laboratory.
Dexter had once again used his doorway in hopes of extracting a new discovery from the 5th dimension. Instead what he got was a comic book detailing everything that was going to happen to him on that day.