It is not likely that this will out perform a modern age computer so the purpose behind building it is scientific rather than technological. Designed in 1837 by Charles Babbage the computer is big (truck sized apparently) and uses steam to run its mechanical parts.
It dates from the time before computers even existed, and a campaign lead by John Graham-Cumming seeks 50,000 people to fund the venture to build a full scale working version of the unit. Parts of it over the last 173 years have been built but never a complete working model.
He plans to take a number of steps should he get the funds needed: digitizing documents originally made and interpreting them, building a 3D model to help with debugging the massive machine and of course building the machine itself!