One thing is certain: interlinking sites doesn't help you from a search engine standpoint. The only reason you may want to interlink your sites in the first place might be to provide your visitors with extra resources to visit. In this case, it would probably be okay to provide visitors with a link to another of your websites, but try to keep many instances of linking to the same IP address to a bare minimum. One or two links on a page here and there probably won't hurt you.
Search engine optimization is a key part of online marketing because search is one of the primary ways that users navigate the web. In 2014, over 2.5 trillion searches were conducted worldwide across search engines such as Google, Bing, Yahoo, Baidu, and Yandex. For most websites, traffic that comes from search engines (known as "natural" or "organic" traffic) accounts for a large portion of their total traffic.