From my perspective and (somewhat limited compared to other contributors) experience, riding a 400 gives you the confidence that you can cover the first stage of most 400s in one go and breaks the physcological barrier of having to ride through the night.
It might also be the first experience of spending 24+ hours awake, let alone on a bike or doing a physical challenge.
In terms of fitness, I am not sure it really helps more than 300s or even 200s. For example, I qualified for PBP in 2007 with the Hailsham 600 and did nothing longer than a 200 for the next 12 weeks. As you know, I am hardly an athlete so if I can do it..... I am dropping the Brevet Cymru for the Hardboiled 300 as a perm.
Personally, I am yet to enjoy rather than endure a 400 but oddly have not found 400k+ a problem on my two 600s or PBP. This suggests that once you've got the miles in your legs, the rest is in your mind.
H