Nowadays we seem to have trains branded Gatwick Express that are neither express, nor dedicated to CVictoria and Gatwick only. So why are they branded in this way? I suspect it's mainly for revenue. Therefore I don't see any need or benefit to passnegers in having these services at all!
They're not branded Gatwick Express, they're branded "express". And I cannot see any reason for the distinction other than a brazen attempt to charge more. Especially when it can be argued that Southern Only tickets must be valid on it.