A 404 page is what a website shows when somebody asks for an address that is not there. Maybe they mistyped it, maybe they followed an old link, maybe a page was deleted. The 404 is the number the server sends back, meaning not found.
The default one is a dead end
Left alone, hosting tends to show a bare white page with a line of technical text on it. No menu, no phone number, no way back. To a homeowner who has just clicked a link from somewhere, that looks like the firm has gone out of business.
They do not email you to say your link is broken. They go back to Google and ring the next firm, and you never find out the visit happened.
What a good one does
Say plainly that the page has moved or gone, in normal words rather than error codes. Keep the full menu and the footer so the site still looks like a site. Put your phone number on it, tappable, because a lost visitor who wanted a quote can still ring you from there.
Add three or four links to the pages people usually want: your services, your areas, your work, and the contact page. That turns a dead end into a junction.
A worked example
A paving firm prints a flyer with a web address ending /driveways. The site actually uses /block-paving. Two thousand flyers go out with an address that does not exist.
With a proper 404 carrying the menu and the number, those readers still find the right page or ring. With the default white page, the whole print run does nothing.