They do so because they want their opinions heard. A poll or survey is an unbiased way for people to make their views known to each other, to their government, to businesses, to educators and many other institutions. This is one way for average Americans to add their voices to the debate over important issues of our day. It is our sense that people want to be included in polls. Medicines in your cabinet became available only after they had been tested for effectiveness and safety, tested among samples of patients.
Manufacturers use sampling methods to guard against producing defective products.
Businesses conduct market research by sampling consumer opinions to help develop products people want and avoid costly mistakes. Even the Bureau of the Census only interviews everyone in the nation once every ten years. The Census Bureau along with other government agencies also conduct sample surveys to obtain social and economic data we see in the news. One thing legitimate survey researchers will never do is try to sell you something. That is called telemarketing.