My short answer is that having the local file on your local machine/haard drive serves to improve save to local times. Consider a 100MB file (or whatever non-trivial size)... everytime you save to local (not central) it Revit (with assistance from the operating system) needs to to reconcile all that data over the network. Keeping it on your local machine eliminates the network latency related to save to local.
It is assumed that you save to local more frequently than STC, so why not make the process as quick as possible?
In large projects, I've also recommended that any link files also be on the local. Consider how often the linked file actually updates (perhaps an update once a week from the architect... of course depends on the phase/status of the project). If you have, say, a 100MB arch file and a 50mb struct file, and you have three separate users (M, E, P), when these three users start working each day, they're pulling 450MB (plus the MEP model) over the network, and this happens every time they open the model. If they copy the A and S files local first, a large amount of 'bytes' may be saved, potentially reducing network contention/bottle necks, etc. Of course, you'd have to be careful to make sure the always have an up-to-date copy of the A/S models, but this is relatively trivial task that can be automated.
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.