Guest posting offers strong links that is available and some blogs also allow to have a contextual link and this is the link type that the Google prefers the most. Considering Guest posting services means you get at the article bottom a powerful link and it offers valuable link juice.
The blogs must be popular so that your link is stronger. In fact, even single guest post links promote the rankings in Google than having low quality thousands of links. Even a quality single link is enough to get good return on investment. Thus, it is more about quality and not quantity.