404 Error is a status message indicating that a requested page or file could not be found.
It matters because development decisions affect speed, security, scalability, tracking, SEO, and maintenance. A site that looks good but is fragile or hard to update can quietly hold back every marketing channel.
Example
Example: If a visitor clicks an old blog link and sees a 404 page, the content is missing at that URL. The fix may be to restore the page, redirect it, or improve the custom 404 page.