NOFOLLOW DOES NOT MEAN DO NOT CRAWL!By Strictly-Software
I have heard it said by "SEO Experts" and other people that to prevent excess crawling of a site you can add rel="nofollow" to your links and this will stop GoogleBOT from crawling those links.
Whilst on the surface of it this does seem to make logical sense, I mean the attribute value does say "nofollow" not "follow if you want" it isn't. BOTS will ignore the nofollow and still crawl the links if they want to.
The nofollow attribute value is not meant for blocking access to pages and preventing your content from being indexed or viewed by search engines. Instead, the nofollow attribute is used to stop SERPS like GoogleBOT from having any "link juice" from the main page leak out to the pages they link to.
As you should know Google still uses PageRank, even though it is far less used than in years gone by. In the old days it was their prime way of calculating where a page was displayed in their index and how one page was related to another in terms of site authority.
The original algorithm for Page Rank and how it is calculated is below.
PR(A) = (1-d) + d (PR(T1)/C(T1) + ... + PR(Tn)/C(Tn))
An explanation for it can be found here. Page Rank Algorithm Explained.
The perfect but totally unrealistic scenario is to have another site with a very high Page Rank value e.g 10 (the range goes from 1 to 10) and to have that sites high PR page (e.g their homepage) have a single link on it that goes to your site - without a nofollow value in the rel attribute of the link.
This tells the SERP e.g GoogleBOT that this high ranking site THINKS your site is more important than it in the great scheme of the World Wide Web.
Think of a pyramid with your site/page ideally at the top with lots of high PR pages and sites all pointing to it, passing their link juice upwards to your site. If your page then doesn't have any links on it at all then no link juice you have obtained from inbound links will be "leaked out".
The more links there are on a page the less PR value is given to each link and the less "worthy" your site becomes in theory.
So it should be noted that the nofollow attribute value isn't meant for blocking access to content or preventing content to be indexed by GoogleBOT and other search engines.
Instead, the nofollow attribute is used by sites to stop SERP BOTS like GoogleBOT from passing "authority" and PR value to the page it is linking to.
Therefore GoogleBOT and others could still crawl any link with rel="nofollow" on it.
It just means no Page Rank value is passed to the page being linked to.