Everyone hashtags, but not everyone hashtags correctly. Aside from being novel they actually perform a job. A hashtag allows tweets to get grouped together. For example if someone is talking about Justin Bieber they might add the hashtag #justinbeiber or #bieberpower or #bieberfever to allow that tweet to show up for people that are tweeting similar things using that same hashtag. Simply put, it is a way of grouping the tweets together to create a conversation. It seems pretty straight forward, right - just make your tweet then add the hashtag that you want to group it with. This is correct, but there are some strategies to consider.
In this scenario you are trying to use a hashtag to group a targeted subset of tweets. The relevance of the tweets are very narrow and you want them to be completely related to one another. In this scenario you might use a hashtag that isn't very popular or is so targeted that noone else would be using the hashtag. You might do this if you are running a specific campaign hashtag so that all tweets related to the campaign can be found with the hashtag - or you might want to do this for a competition or event. An example of a hashtag like this might be #e32015 which is the hashtag for e3 convention for 2015.