HTTP is ubiquitous now with pretty much everything being powered by an API, a web application or some kind of cloud-based HTTP driven infrastructure. With that HTTP Security becomes paramount and to secure HTTP you have to understand it.
HTTP is the protocol that powers the web and to penetrate via a web service it pays to have a good solid foundational understanding of HTTP, how it works and the common response codes – many of which can lead to some kind of vulnerability which is exploitable.