I'd argue that legal immigration is changing the US more than illegal immigration because illegal immigration largely originates from other countries in the Americas and therefore more easily fits in with the USA's existing history and culture.
low cost, low overhead, no risks due to INS rounding up your people, and no tariffs due to NAFTA.
the reason they want the in the US is to work in services to keep costs down, because overall wages have not gone up and the costs of everything domestically needs to trend down -- and that means cheap labor.