It is the cheapest way of getting into contact with your audience. You do not need to build a website where people can react.
It is the cheapest way of reaching a bigger audience. Everybody following a person using the #hashtag will get a message and maybe some of them will be interested in the subject of the hashtag. So you do not need to buy airtime or advertisement space to get your program out there.
It is the easiest way of getting people interested in your program or subject because others are doing all the work of promoting your program.