Please note that all of the options used and changes made to the html meta tags are done so for allowing or preventing the search engines and certain sites' robots files accessing certain areas of your sites. You know, they cannot do some changes to the way your pictures for instance are being displayed when people arrive at your website because such major changes need the editions of the codes that make up the actual body sections of the sites and not the tags displayed online. If you look at the homepage sections of the sites online, you can notice that more than one single way of communicating with the search engines robots tags are being used and this shows that search engines have broader means and ways of accessing a site's contents and sections and they do not limit themselves by following some tags created and added by webmasters, you know. For example, if you want to test the case for experiencing yourself, try omitting all of the robots tags from your site's pages for several weeks and see what happens on the next sessions search engines crawl your site.
Well, nothing serious will happen but do you know why? Please note that although robots tags are important, but search engines are regularly crawling the web and visiting as many websites and internet pages as they can for finding and indexing fresh content pages and useful information. For this reason, even if there are no html robots tags used in your web pages, they still will be fully crawled and visited by the search engines softwares and scripts and you can rest assured that your great web articles and content pages will get fully indexed by all of the search engines but using the html tags can definitely boost the indexing process of the sites and web pages. You know, it's just the matter of helping the search engines via different routes and sections for finding the pages that are parts of your site online. Naturally, every webmaster likes seeing more and more pages of his/her site getting indexed in the search engines and main internet portals and knowing how the html tags can be helpful in this regard is highly recommended. The homepages of the sites are better using the "ALL" indexing format of the robots tags so that other pages connected to the homepages too get crawled and visited.