Google assumes that your site will have a hierarchal structure, and its algorithm is built on that assumption. This starts with your home page at the top of the hierarchy. Then your global navigation (the links that appear on every page of your site, in the header and footer usually) should link to the next most important pages. Those should link to the next level down, and so on.
As a general rule of thumb, every page should link to the top layer of the hierarchy (the global nav) and each page should also link to the pages above and below it in the hierarchy. For larger sites this is where breadcrumbs come in useful: go to the product page of most ecommerce sites and you’ll see a list of links showing the hierarchal path you took to get there. These breadcumbs provide another crawl path for bots, as well as flowing link authority up to more important pages.
This hierarchal structure is very important for ranking, as well discuss in Authority: On Site, but it’s also important for crawlability. Googlebot generally crawls into your site from an external link: that link may point to your home page, or it may point to something at the very bottom of your hierarchy. Googlebot will continue to crawl through the links it finds on that page and subsequent pages; however, at some point it’ll stop.
A strong hierarchal structure will make sure that regardless of where Googlebot enters your site, it’s definitely going to crawl the most important pages. If anything on your site gets skipped, you want it to be the least important ones.
As you can imagine, it’s vital that every page of your site needs to be linked to from somewhere within that hierarchy. A page is only going to get crawled if another page somewhere on the web is linking to it.
By Definition Hierarchy is Not Flat
It’s worth stressing here that you should not go crazy and embed hundreds of links on every page linking like crazy to every other page. This will do bad thing for your authority, as discussed later in the authority sections, but it also removes the hierarchal structure of your site.
Remember, Google expects the most important pages to be linked to the most, and the least important to be linked to the least. I know that some business owners think every page of their site is the most important, but if you link to everything equally, you are creating a flat structure where no pages are important in Google’s eyes.