Several Win32 APIs and .NET platform components (such as WCF) use the HTTP server API when they want to send or receive HTTP requests destined for the local computer. The HTTP server API basically provides this functionality in an OS-driven manner, without having to deploy a stand-alone web server such as IIS on the machine.
At this point, it is best to quote the Dev Center page linked above:
Reservation constantly allocates part of the URL namespace to individual users, allowing them to reserve or “own” that part of the Namespace. Reservations give the user the right to register service requests for the namespace. The HTTP server API ensures that users do not register URLs from parts of the namespace that they do not own. To ensure the security of the namespace, an ACL (access control list) is applied to the part of the namespace reserved for each user.
Reserved namespaces are identified by URL prefix strings, formatted in the same way as URL prefixes used for registration. This means that all the various categories of host qualifiers are also available for reservation.
Reservations of the namespace are preserved during reboots, and the changes are dynamic, so there is no need to stop and restart the machine.
This means that before the HTTP Server API allows you to listen for incoming requests in a specific URL namespace (think of it as a "URL path"), you need to register for them. Registration is based on the user account, as indicated above, therefore, the user account is important here, according to which the process that wants to listen to the execution of the request may differ from the current user account.