A dead page caused by an incorrect or out of date link, 404’s are most commonly caused by dead links.
Things that could cause a 404 error
The user has attempted to navigate the website manually by typing in the URL manually and has mistyped or guessed incorrectly a page.
Web page has been moved or deleted
Admin has misspelled the URL in the navigation or linked image etc.
Fixing 404’s is vital for improving any websites SEO as they count as negative factors in the ranking algorithms.