In my opinion,
Some exists as purposeful duplicates in case one site goes down, through legal or whatever other means. Others might exists solely as a money scheme, especially if they are overly similar to another; which could even possibly be the result of sharing the same database, pretty much creating nothing more than a re-skin. After all, more sites equals more viewership, which equates, roughly, to more people accidentally clicking on ads. If you notice, most of those that seem like knockoffs still follow the same naming convention, especially with having the keyword 'novels' in their name. Kinda making it more likely that someone googling for free novel sites will almost be guaranteed to find one or the other.
Creating a similar site to a previous one, especially a novel hosting site, should be a rather easy thing to do if you've already got the systems and structures in place from the former. Plus, you won't need to recreate those aggregator bots.
Anyway, that's my two cents.
