Hashtags make it possible to group such messages, since one can search for the hashtag and get the set of messages that contain it. A hashtag is only connected to a specific medium and can therefore not be linked and connected to pictures or messages from different platforms.
Because of its widespread use, the word was added to the Oxford English Dictionary in June 2014, defined as: hashtag n. (on social media web sites and applications) a word or phrase preceded by a hash and used to identify messages relating to a specific topic; (also) the hash symbol itself, when used in this way.