You may have not thought about forward Thomas Robinson and immediately thought he would be a factor in bringing in Dwight Howard for Houston. His trade to the Blazers doesn't say "blockbuster," but it certainly helps if you're trying to bring Superman in.
Robinson had a $3.5 million contract, so clearing him gives Houston enough space to offer Howard a max contract. That ability to offer a four-year/$88 million contract is certainly something to bring to the table when the Rockets' representatives meet up with Howard.
UPDATE: Houston is also open to trading Jeremy Lin and Omer Asik. Asik wouldn't be needed if Howard agrees to sign with the Rockets, but Houston will have to search for a new guard if Lin goes.