One of the good things about trams is that they are very good at loading/unloading passengers quickly. Currently our 'network' doesn't make very good use of this, because at the point where there is most congestion (Princes Street) there is either nobody getting on (eastbound) or nobody getting off (westbound). Having a route down as far as the bottom of Leith Walk would mean we can get a lot more passengers onto Princes Street without adding any stopping time on Princes Street.
I hope this would mean they can get rid of a couple of bus routes, but that remains to be seen.