Comment by gnosis

Making a SO competitor is not the only reason to download the data, just as making a Wikipedia competitor is not the only reason to download Wikipedia data.

One very important reason for downloading the data is for simple archiving. If Wikipedia or SO disappear tomorrow, their data will still be in the hands of the public, ready to rise like a phoenix from the ashes. Not so with proprietary, closed data that's never released to the public.

Another important reason to download the data is to use it locally and with your own tools. Plenty of people download Wikipedia data for offline browsing. I'm sure the same could be done, at least in theory, with SO data.

Yet another reason is to mine the data in ways that the parent website might not allow (due to lack of knowledge, lack of interest, or lack of manpower).