Why broken links matter
A link that leads to a 404 is a dead end for visitors and wastes the crawl time search engines spend on your site. Broken internal links also stop rankings from flowing to the pages that need them. Links that redirect still work, but pointing them straight to the final address is faster and cleaner.
How the check works
Clarnic reads the page, collects its links, and checks up to 40 of them (internal links first) with one request each. Each link is marked OK, redirect, broken (404 or another error) or blocked. Some sites, especially social networks, block automated checks with 403 or 429, so those are marked as blocked rather than broken.