The multiplication of imprints is part of the so-called "death of the midlist." What's happened is that rather than one big list with all the books from one publisher on it (frontlist being the ones at the beginning of the list, backlist being the ones at the end of it, and midlist being everything in between), the books are broken up into multiple lists from multiple imprints, so more and more of them have a chance to be frontlisted on one or another imprint's list.
Thus, suppose Dreadful House (made up name) normally published thirty titles a year. The sales rep would come around, and the bookstore buyer would say, "Give me three copies of the top five books on your frontlist, 'kay?" So, five titles from Dreadful House hit the shelves, and twenty-five midlist writers are sad.
Then Dreadful House creates a couple of imprints, Dire House and Drool Press. The sales rep comes around with three lists, each with ten titles (plus backlist, which would be everything else they currently have in print). The sales rep whips out the Dreadful House list, and the bookstore buyer says, "Give me three copies of the top five books on your frontlist, 'kay?" Okay! Done!
Then the sales rep whips out the Dire House list, and the bookstore buyer says, "Give me three copies of the top five books on your frontlist, 'kay?" Okay! Done!
Then the sales rep whips out the Drool Press list and the buyer says, "Give me three copies of the top five books on your frontlist, 'kay?" Done!
And fifteen writers are now happy.
The midlist is vanishing! Oh noes! Ten of those writers, who would have been midlist writers, are now frontlist writers.
When looking for places to send your works, look at the guidelines for each imprint.