Tags are all about Search Engine Optimization (SEO) which is basically the website/blog is represented on a search engine ranking. In SEO content is king thus the more content relevant to the blog’s subject the better as when a search engine goes through a website it logs all the words taking into account the most commonly used words. For example, if a post’s subject is on photography then relevant tags would be; Canon, Nikon, Photographs, shutter speed, aperture and so on.
When a search is generated from search engines such as http://www.google.com or http://www.bing.com the keywords in that search will and can link to your blog. Tags are a very easy and helpful way of improving SEO as they are are keywords of a page/post thus boosting the sites ranking when searched.
Relevant Tags will be used on every post on this blog to aid SEO.
Note: Search engines cannot read images which is why it is important to put alt tags on images when uploading to a page. It is also the main reason why Java Script (JScript) is becoming more popular than Flash because Flash cannot be read by search engines where as JScript can.