If the books are standalone, then yeah numbering might be unnecessary but for ongoing series like Harry Potter, where the events of the books all build off of each other, it is nice to have a quick and easy way to know which order they go in. I know you can find the information on the internet, but it's still nice to be able to look at the cover and know instantly which order they go in.
In a pinch, you can look at the copyright dates, but that only works if the books are being written and published in chronological order. And, yeah, with a continuing series, it can be a struggle to figure out what order to read them if the covers don't make that clear.
Sale-wise, however, it can argued that numbering books discourages new readers from picking up the latest book. A blurb identifying a novel as "Book Five in the Florida Cat Detectives Series" pretty much tells potential buyers not to bother unless they've already read Books 1-4.
Which is fine if Book One was a monster bestseller, but if you're still trying to build the series . . .