Anyone who worries about plagiarizing online, needs only to file the right copyright for their work by registering at the copyright office. That is a sure way you can take a case of plagiarism to court. That's if, someone is foolish enough to copy that creator's work and claim it's his/her own instead. Which is not that common with self publishing individuals. Especially with indie writers and comic artists.
These mediums strike more copper then gold. No offense, I'm a writer and artist too. Despite the adversity in this industry, we all have a good air of insanity to us to believe 'yes we can make it.' Publishing online, is now the best way to truly know how far you can go. As to take a chance with a traditional publisher in the twenty first century without having a known name is worse then playing the lottery.
It's not true, that writers don't know what it takes to get published. Writer's digest puts out a great guide every year for a long list of publishers for what it takes for each specific publisher. That improves the odds only enough to prevent your work from being thrown out after glancing the first page. That's the medium we continue to write in. So, there's a lot to appreciate for a writer who gives it his/her all online by publishing online, copyrighting, and marketing.
And there is potential profit to be made if your good at it. Different ways online to make some cash online with your fiction. It's also fair to note, traditional publishers will from time to time publish good online fiction. There's evidence of this online.
Still, it's good for any creator to find someone to give them some humility. Using proofreaders and editors that will review their work without having automatic fan syndrome (friends & family). Surprisingly, many writers cannot stand criticism. A good writer however, will preservere and use most or all of that feedback to create a better product. Writers and artists can also use the feedback gained from publishing online to improve their work. So, that's a few of a long list of reasons why to publish online.
If nothing else, it can create a sense of accomplishment that is lacking by having it sit on the hard drive for years.