Tags are helpful for search purpose if you submitted your blog a site some visitors searched a particular keyword which you inserted has a tag then the sited will show your blog to users..

It does not matter but its a good practice if you use as they also get indexed on search engines.

Tags are the good source for ranking and many people search through keywords. Tags are indexed by search engines.

Make tags for users, it will improve your rankings, because tags are indexing by the search engine. However, do not forget the Panda - for search engine spam you could be punished

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Tags are very important if you want to be seen. These are your keywords and will help search engine determine your site’s relevance to user searches. When people search for something on Google they usually enter keywords or phrases of what they want to find out. If you have relevant tags to your content then Google can easily bring up your site when searches from people match your tags.

It is good practice to help both people who use search on your website as well as makes life easier for Google's algorithm.

Tags screw up site structure with redundant internal pages.

Forget Tags, & use parent/child pages (silo).