Looking at 12,000 crawl errors staring back at you in Webmaster Tools can make your hopes of eradicating those errors seem like an insurmountable task that will never be accomplished. The key is to know which errors are the most crippling to your site, and which ones are simply informational and can be brushed aside so you can deal with the real meaty problems. The reason it’s important to religiously keep an eye on your errors is the impact they have on your users and Google’s crawler.
Having thousands of 404 errors, especially ones for URLs that are being indexed or linked to by other pages pose a potentially poor user experience for your users. If they are landing on multiple 404 pages in one session, their trust for your site decreases and of course leads to frustration and bounces.