This discussion on Kartbook lead to the question "why move the seat back when the track gets grippy?", so I'll have a crack at answering that question here.
Firstly, let's define something here. When I say "the track gets grippy", I don't mean "the track is an unmanageable mushpit", I mean grippy. That means the tyres have got more grip to work with, and faster times are possible. People often describe, hot, mushy, crappy, slow tracks as "grippy", and that's not really what I'm talking about in this instance.
So, why move the seat back on a grippy track?
Firstly, the more grip there is available, the more rapidly the front tyres will direct forces into the kart when you turn the steering wheel. That means the chassis gets excited more quickly on corner entry, and wants to start lifting a wheel sooner. Moving the seat back helps to dull that initial excitement in the chassis.
Next, as the outside rear tyre starts to get involved, the weight shifts up and forward from the inside rear. If that transition is too rapid, the chassis might start bucking, or simply lift-and-drop, resulting in either a slide or a bog, both of which are slow. Having the weight further back controls that rate of lift, and keeps the weight transferring at the correct rate.
At corner exit, in theory more weight over the back is probably a bad thing, but remember that 95% of your corner exit speed comes from how your kart negotiated the entry and midcorner. If moving the seat back means the wheel has lifted smoothly, slowly, and stayed pretty low, that means that as you get on the throttle again, the weight will transition back smoothly and slowly as well - and that's where you get good exit speed from on a grippy track!
Incidentally, a good test to see if the track is heading in the direction we're talking about is if the kart is faster when you reduce the caster. If less caster is working better (because it's helping the kart lift the wheel slow and low), then moving the seat back might be a good move. Sometimes you should leave the caster at the greater position and move the seat back instead, though that's mainly dependent on driving style.