What I've discovered from using Submittable:
1) Many journals have submission fees, so submitting in bulk can cost some money. The lowest fee I've seen is $5.00, the highest fee I've seen is $30. The more "literary" or exclusive the journal, the higher that fee.
2) There are a lot of themed calls for submissions -- fantasy, horror, romance, cross-genre and more. Some of these offer a prompt -- one of the ones I entered had the prompt "Catch up".
3) I have a ten percent success rate, which has kept me from the despair about not finding an agent/publisher for my novels.
I get a lot of rejections for my work, but because there's always more contests, and more hope, I feel better about trying.