As others keep saying, guest posting works, and will always work if the site is real, has real traffic, and is relevant to your niche. If it's on a prestige site, it should add trust and reputation to the writer or the webpage it points to. One can also use them to piggy back on the situation where somebody wants a gig or freelance writer to do a post related to your niche. If you are ranking your site on topic X, while advertising you write and publish gigs on topic X on real sites, you can work out a deal where you get paid to write posts with links to both their page, and yours.