Mastodon is your personal soapbox where you can shout your thoughts.
However before you have followers, you're kinda shouting into the void.
However hashtags are like little campfires and if you go to one. You'll see people. If you share your thoughts around the campfire, people will see you.
Now once you've met people (and followed them), you can see them from atop your soapbox, no need to stand around the fire. Though in some cases, you won't know what they said before you became friends.
The more campfires you find (and follow) along with people, the busier your home feed is and the more interesting and/or fun your experience will be.