Why do some pianos have the bass bridge connected to the long bridge?

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<< Why do some pianos have the bass bridge connected to the long bridge?
I've heard this also called a "ring bridge".
 So I'm wondering why this connection is used and why only in large pianos? >>

Greetings, 
   Umm,  this is my shade-tree thinking on the subject: 
The continuous bridge does several things.  
    It creates a more direct connection between the strings of the bass and 
the treble bridge.  This allows a more direct sympathetic activation of all 
unstruck strings when the damper pedal is depressed, thus making more efficient 
use of the energy in the struck string, ie,  the struck strings' energy travels 
via the maple bridge to the other bridge without having to travel into the 
soundboard, the ribs then over to the other bridge. 
    It is a stiffer bridge, since the triangulation principle is in effect.  
A stiffer bridge creates more sustain. 
    It reduces the difference in impedance seen by the strings on the 
separate bridges, helping to ameliorate the bass break discrepancy.  
    It is certainly a sales asset, since the average buyer will have no idea 
of the meaning of the above. 
Regards, 



Ed Foote RPT 
http://www.uk-piano.org/edfoote/index.html
www.uk-piano.org/edfoote/well_tempered_piano.html