Good internal linking helps both visitors and search engines. This crawler walks up to 40 pages of a site and reports broken internal links (with the pages that contain them), links pointing through redirects, sitemap pages nothing links to, click depth from the homepage and per-page in/out link counts.
They waste crawl budget, leak link equity and frustrate visitors — search engines treat many 404-links as a quality signal.
A page that exists (e.g. it's in sitemap.xml) but no other page links to it — crawlers rarely find and rank such pages.
It keeps the check fast and the load on the target site polite; for most small and medium sites that covers the important structure.
How many clicks a page is from the homepage. Pages deeper than 3 clicks get crawled and ranked worse.