So far, the examples you've seen have been concerned only with finding chapter headings wherever they occur. Any occurrence of the string 'Chapter' followed by a space, followed by a number, could be an actual chapter heading, or it could also be a cross-reference to another chapter. Since true chapter headings always appear at the beginning of a line, you'll need to devise a way to find only the headings and not find the cross-references.
Anchors provide that capability.
Anchors allow you to fix a regular expression to either the beginning or end of a line. They also allow you to create regular expressions that occur either within a word or at the beginning or end of a word. You cannot use a quantifier with an anchor. They're definitely not the same. Not only does a true chapter heading occur at the beginning of a line, it's also the only thing on the line, so it also must be at the end of a line as well.