A 404 error happens when someone opens an address where nothing is - the page was deleted, moved, or the link has a typo.
Why fix it
A few 404s are normal. The problem is when links from your own site, your ads or other sites point to them. The visitor hits a dead end and the link’s SEO value is lost.
How
When addresses change, set up a 301 redirect from the old address to the new one and check errors in Google Search Console regularly. The 404 page itself should offer the visitor a way forward.
