Originally Posted by elphenor

You use it when you intend to use the data to figure how individual groups voted

let's say you conduct a poll of 1000 Americans, that is a large enough sample size to produce a small margin of error, but mathematically you may only end up with 100 Hispanics for instance not a large enough sample size to get a reasonable margin of error for that specific group

So if they plan to use the data that way they will oversample smaller groups like Hispanics so they get a reasonable amount and then weight them back down when they use the data for other purposes

Sampling | Pew Research Center