The purpose of the hashtag is simple, to allow people to search and participate in relevant conversations. It also provides a purpose far greater than just the search function, it allows social media websites & search engines to identify popular topics. For example, when a popular TV show like XFactor loses a fan favourite, people immediately flock to #socialmedia and thrash out the hashtags. Social media algorithms analyse the trending hashtags and this changes how sites display their content.
Hashtags are important as they provide an easy form of identification about your post however it is not the be all and end all of social media. Even if you do not include hashtags and your content is original, creative and topical, there is every chance that it will go viral.
Check out some interesting statistics about engagement and the effective use of hashtags in posts