From the Purple Beast's website:
It is in peoples’ best interest to be counted because the US Census is how our country makes decisions about allocation of resources and local representation, regardless race or immigration status.
Here is Beck on the topic.
Why the effort to include illegals?
Well, very simply, if you didn't watch the video above, it goes to the redistricting of Congressional districts and the apportionment of the treasury.
More bodies (legal or illegal) mean more representatives. More representatives for illegals means more votes to fund programs, more votes to confer rights, more votes to do the SEIU's bidding.
For more info on the SEIU, go here.