There is no boost in SERP placement as a result of using mark-up with one round-about exception. I will explain.
A page has mark-up and a search query makes a match against that page. Simply because the matched page has mark-up, one cannot assume there is a boost in SERP placement. That is not how it works and an overly simplistic view. There is no metric and/or algorithm that will boost a pages placement simply because a page has mark-up.
However, answers from the knowledge graph can influence trust for the match over others and early search query algorithms may place any match found from the knowledge graph over others. Mark-up is not necessarily a requirement for the knowledge graph since the knowledge graph predates mark-up, however, it is a tool for trusting the data from content. One requirement for inclusion into the knowledge graph is that the data is verifiable/corroborated in more than one trusted place. Please keep in mind that this is extremely early in the search query process. What has to be remembered is that are quite a few algorithms that influence SERP placement and can ultimately remove entries from the SERPs including filters that effect SERP level penalties. So again, mark-up in of itself does not influence SERP placement, however, matches from the knowledge graph that may be a result of mark-up may influence placement.
Pure and simple, mark-up is not available to all websites. For example, there is (last I looked anyway) no mark-up for whois and other network information. There are other examples primarily in the data presentation market. Mark-up is targeted to sites that can benefit most such as e-commerce, business, organizations, etc.
Be that as it may, mark-up is still a good idea. Mark-up is intended to communicate data directly to search engines in a way that can easily be understood without having to parsing pages. Parsers are difficult to write and can fail from time to time due to unseen exceptions. While Google has gotten it right for a long time without mark-up, mistakes are still made. I saw a few just yesterday! Mark-up is your opportunity to be unambiguous. It is wise to take the opportunity even if it does not appear to be needed.