Why the last item should have no URL#
Google's own BreadcrumbList documentation is specific on this point: the final item in the trail — the current page — should omit the item (URL) property, since a page linking to itself provides no real navigation value and Google explicitly does not require it. Most hand-written breadcrumb schema fills in every URL out of habit, including the current page's own address, which is not wrong exactly, just unnecessary. This tool leaves the last item's URL field optional specifically to make the correct, minimal version the easy default.
Why position numbers are assigned automatically, not typed by hand#
Each item in a BreadcrumbList needs a position field — a 1-based index matching its place in the trail. Typing these by hand is a common source of off-by-one errors, especially after reordering or deleting a level partway through building the trail. This tool recalculates position automatically from each item's actual place in the list, so reordering never leaves a stale number behind.
This describes navigation, not the page content itself#
BreadcrumbList is one of the few schema types that describes how a page fits into a site's structure rather than describing the page's own content — it is metadata about navigation, meant to match the visible breadcrumb trail a user would actually see on the page (Home > Blog > Article Title). It is usually paired with other schema (an Article or Product type) describing the content itself, not used as a replacement for it.