Snowdrop is the name of the flower, just like rose, tulip, hyacinth.
A journalist is someone who writes journals for newspapers etc. It is not the who he does it for, but what he does. A journal is nothing more than a record of events.
When we say writer, we usually refer to someone who has written a book or at least writes for his profession.
When we use writer followed by something specific( a.e. writer of this letter, writer of this note) than we are just talking about the person who wrote this or that.
I hope this helps.