Selected answers from the Dumb SEO Questions G+ community.
Ash Nallawalla: I fix them because they usually matter to human visitors.
Mark Walker: Yes, in my experience they’re quite important - especially 404 errors which google seems to hate
Stockbridge Truslow: Keep in mind - when something actually DOES go away (and isn`t just moved to another URL), it SHOULD throw a proper 404. That`s what it is there for. The warnings from Google are simply there to say either "This was here but now it`s gone..." or "We`re finding links to this, but it doesn`t seem to be there." Google has no hatred for this - it`s just giving you an alert so you can confirm that it`s actually gone and not moved or misspelled or whatever. You should never arbitrarily redirect a missing page somewhere. If it has ACTUALLY moved to a new location, then redirect the old URL. If you, for example, combine the content from two different pages into one, you can redirect both of the old ones to the new merged page. In most other scenarios (though I`m sure someone may come up with a few other good reasons) the 404 is proper. NEVER NEVER NEVER redirect a removed page to your home page. And never redirect an old page to a new page that doesn`t have the same message and meaning as the original page. That practice was from back in the olden days when PR was the biggest ranking factor and you wanted to keep it from being lost - you could funnel it back to another page on the site. Nowadays though, that raw PR is important, but there`s also a more important contextual value to a link (and the page it`s linking to). So if you have a page about widgets and you redirect it to a page for left handed smoke shifters, you`re hurting yourself.The existing links on the widgets page and the content itself is saying, "I`m a page about widgets." The redirected links, on the other hand, are sending the signal that, "I`m a page about smoke shifters." Google is then going to look at it and wonder, "Is this about widgets or smoke shifters? I am not sure, so I`ll rank this page from another site ahead of it since I know what that page is about."
Mark Walker: When something does go away it should throw ‘http 410 - gone’, not 404. Otherwise google and your users are left guessing.I’m not convinced google don’t dislike 404s either, they certainly don’t help rankings in my experience.
Stockbridge Truslow: True. 410 is the correct way to handle a page that is totally "gone for good." I omitted that in my post above because it would make a complicated subject even more complicated and also because there is very little difference in how Google treats a 410 over a 404. About the only benefit you get from using it is that it`ll get the page off your 404 error page. A benefit for not using it is (if you are keeping track of things well) is that if you get rid of a page with certain content and then bring that topic/page back to your site at a later date, you can often still redirect the old URL to the new and potentially reclaim some of the ranking power which may still be lingering (if any). Again - these are a whole set of different nuances too complex and nearly irrelevant to the particular question at hand. As far as 404`s helping your SEO - well, of course, having something not there couldn`t possibly help SEO. That would make no sense. But you would rather throw a 404 than make an arbitrary 301 to some other page on the site that isn`t actually the same subject matter. That will HURT SEO over the long run.
Faith Dugan: Stockbridge Truslow the problem is I have a 4xx error but the page does not exist anymore and the url is not relevant to any content because I have installed the demo version of a theme (newspapertheme). I googled and come to a conclusion that I have to "mark as fixed" in the search consoler under crawl errors section .. ?
Stockbridge Truslow: Yeah - that will get it out of your 404 error list. That said - you SHOULD make sure there are no links on your site that still refer to that page, too.
Perry Bernard: If the page was a theme template page it has no relevance. Shame on your developer for leaving the page there when you allowed Robots. Kill the page, mark the 404 as fixed in GSC, move on. Usually little if any harm done aside from possibly sending user traffic to a bogus theme page while it was there.
Rob Woods: As above. If it has external links, internal links, or traffic. If it’s a url that never existed or one that legit went away and isn’t linked to cleaning them up is more of a housekeeping issue than an SEO one.
Rob Woods: Or if it has internal links, fix those as well so they don’t point to a 404 url