When you host a website your domain name has to resolve to an IP address so your computer can connect to that IP address and load the websites contents. If you host your website on shared hosting then you will be sharing the same IP with serveral other websites. Anywhere from 10-10000. Also even if you don't have shared hosting there is a strong chance that the dedicated IP address will of been used by a website your hosting provider was using before you used it for your website. How do you know these websites weren't hosting Malware or spammy websites that have been deindexed by Google? This is what Bad Neighborhood Checker can tell you.
Everytime the search engine Bing crawls a website it remembers the IP address its hosted on. Using advanced search modifiers we can scrape a list of websites that are sharing the same IP address as your website currently does. Once we have this information we can then run checks on the website to see if you really want to be sharing an IP address with that website. For example Bad Neighborhood Checker will check the website for malware, foreign content, spammy content farm, cloaking and if its been deindexed by Google.
You never know who you're sharing an IP address with or who owned it before you if its a brand new website. So its always best to check. For example you may of just thought you got a great deal on some super cheap hosting but so did someone who wants to host their PBN there. So you end up sharing an IP address with 30 PBN websites. Now when they all get deindexed there is a strong chance Google may decide to automatically deindex your website too! Unfair I know, but Google is very "proactive" in its anti-spam algorithms these days. So why take the chance?