From NPR's "planet Money" blog. Most countries let people in to be workers. The US legally admits people mostly for family unification:
http://www.npr.org/blogs/money/2012/08/27/160110929/immigration-who-the-u-s-lets-in-and-why?sc=nl&cc=pmb-20120827
A couple of things to point out- Most undocumented immigrants come to work, not to live off the taxpayers. There are actually precious few resources for udocumented immigrants. Of the documented immigrants though, the fact that people are let in to unify families means that a lot of spouses and children (dependents) are let in to accompany people who are already legal residents and are pretty much self- sufficient. These aren't necessarily people who show up and then start living off government services. It also takes an average of five years for people to gain admittance to the US as part of the family unification program. It's also a way to attract people who have skills such as surgeons and engineers- if they can't bring their families eventually they won't come. On the other hand, the more people we let in specifically to work, the greater is the competition for jobs in the US.
