What's the benefit of taking unskilled people in who's backgrounds we don't know?
The benefits are:
1. It depresses wages for the poorest Americans. The American left used to be vocal about this 20 years ago, before the left turned into "we need more transwomen of color exploiting the poor and bombing brown people! [Sponsored by Google and Microsoft Bing]"
2. It creates new Democrat voters within a generation. If you can think 20 years down the road (and they have) then this is a near-infinite source of voters, consequences be damned. It's not like these illegal immigrants are gonna live in the gated communities of the rich and powerful.
Democrats like it because of #1 and #2. Before Trump, Republicans liked it because of #1, and they still do, but now they can't say it out loud anymore because it will piss off the younger Republicans who are more protectionist and less libertarian.
Oh, did you mean benefits for Americans in general? Don't be silly. Illegal immigration benefits no one but the 1% and the illegal immigrants.
Drives down wages for capital owners while driving up demand for assets like housing, that's not the reason though, billionaires and corporations are just super compassionate towards migrants that aren't in Martha's Vineyard.