I think the best reason to have your own website is so you've got everything there, archived, backed up, and in place for those times when the platform you're using fails. (You get banned, they vanish overnight with no notice, you decide to migrate for totally different reasons, etc.)
I certainly agree that you should be focusing on whatever your most lucrative platform is. However, since it's not actually "your" platform and you're subject to their T&C, they could just leave you high and dry one day and if you don't have something in place to immediately direct your fanbase to, it becomes an even worse headache for you.
Lastly, having your own page allows you to focus on more than just comics or novels if you create additional things. You also have much more flexibility over what content you are allowed to post.
So my advice is to have your own website (it can be a free wordpress or tumblr site, doesn't really matter) for backup purposes. And needless to say, have your own offline backup of all your files and text, because even wordpress might vanish one day. You just never know.