Why Free Guest Posting

Free fresh content

As the others have already pointed out, it’s a free and easy way to get content, but there’s more.
Big sites often run the risk of growing stale as they uphold their brand. Free guest posts are one way for them to shake things up and allow for some fresh ideas and perspectives, without the danger of compromising the brand they worked so hard on. After all, it’s someone else’s post, right?
While not every writer has a large pool of readers, most do have some audience, and those people will read their guest post. That means a steady trickle of new visitors to the site, many of which may decide to check out the rest of the site’s content.
There’s also the benefit of backlinks and networking. Staying connected is never a bad thing, so the site and the guest poster will essentially help each other with ranking.
Of course, allowing free guest posts does mean that the site will need an active quality control division. Bad content is always worse than insufficient or no content.

It is a win-win for them, they accept quality content which helps them rank and increase traffic while the guest gets a link.

It builds the relationship with other bloggers, also it helps to add the fresh content.

Constantly adding new content to your website is good for SEO and probably the article author gets a free backlink in exchange.

Authority sites do this to show gratitude to their visitors, other blog owners do for free fresh content, it helps both people. Some owners do link exchange through the free post.

I do this and it's really a win-win situation based on my experience

New Contents will help SEO.

Google loves that so bloger owners are open those.

It will help to get free content.

any such sites examples, please? I'm ready to fill them with fresh content!

Well, it is a symbiotic relationship. You get a link from a site with authority, and he gets quality content from you. And if your site is also of good quality, it means Google will love the fact that he is linking to quality sites.