Have you ever felt the need of redirecting 404 errors to your homepage, or some relevant contents?
Doing these actions is extremely bad for the internet and for your website visibility.
The 404 is there to explain to the user and the search engine that what it was looking for doesn’t exist.
And I think this step is pretty clear to everyone, but, what to do then after you reach this point? You don’t want that the users leave the website without looking for something they might be interested into.
As obvious as it will sound, you want to keep them there, you want that the users find the 404 page, you want that they know that it doesn’t exist, you want that from there they reach something else.
So, spending time and putting effort into creating a custom and rich 404 page will pay you back for each 404 error you may have.
As explained better here, improving that side of your website is important as developing meta descriptions.
Also, as general purpose guide, Google released an handy guide that explain some many tricks about the Search Engine Optimization.