It helps people unfamiliar with the network. Even for those familiar with metros, having two central branches is unusual. For people from less urban areas I bet it makes even less sense. I wonder if TfL have any data for the number of people who get on the Northern Line at Waterloo expecting to get to Old Street or the number getting off at Tottenham Court Road who want to go to Moorgate, for example. Plus since there is not distinction in the audio announcements at TCR and Bank, how could someone who's partially sighted or blind be expected to figure it out?
Yes, in recent years I have occasionally encountered visitors to London who've been a bit confused when using the Northern Line; but they've all been people using their phone to navigate rather than looking at a tube map - as available to pick up at every station. I've never found anyone having difficulty if they have a tube map in front of them.
Also, given that all combinations of movements at Camden Town's junctions are possible without any conflicting movements, then I'd reckon it'll be precisely when the line's signalling and rolling stock are upgraded (and hence scheduling can be more precise) that a mix-and-match service at Camden Town will cease to be any problem.
For Londoners, having the option of both northern branches going via both loops through the centre is a great boon.
If Camden Town had appropriate cross-platform interchanges both northbound and southbound, like at Kennington, then it wouldn't be so bad. But since it doesn't, then unless the junctions at Camden Town made a large proportional difference in possible headways and so on, then I prefer the pattern as it is.
And even if, at some times of day, there is an operational separation of routes, if you "officially" separate them it takes away options at times when you really need the flexibility to switch routings. I think the current system, which Londoners are well used to, and which often gives the option of a through train or a slightly quicker take-the-first-and-change alternative, is less confusing than if there were two officially separate lines which occasionally mixed across with one another.
We have a system which, for historical reasons, isn't as tidy - or as boring! - as it might be. I'm fine with that.