Depends on the content and Copyright laws, you can't just paste info in and not give the source. If everyone did copy&paste of all info the Internet servers would soon end up bloated with repeats of same info and grind to a halt.
Q. How many sets of the same reference books does a library need to cater for it's customers?
This is a big subject, but in essence it helps build traffic on a website if you can do what's called reciprocal linking. You put a link to site A on your site, and it has a link to your site. One of the ways Google uses to rank a site in search returns is by looking at the number of inbound links to it.
That said, if you sell purple dog food there's not much point in linking to other sellers. Your link might be to a site selling purple dog food bowls.
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