this the only meaning I could find why
The base element allows the URL of the document itself to be set, to help browsers in situations where the document may be read out of context. It is especially useful in allowing browsers to determine any partial URL's, or relative paths that may be specified (for example, in <A HREF> elements, or paths pointing to <IMG SRC=...> (images). The <base> Element should appear within the bounds of the <HEAD> element.