Staged boiler offer several advantages.

Redundancy - No Emergency calls.  One boiler can handle almost the entire building and all the hot water for the home or business. 

Greater Efficiency - Running one boiler at a time allows for the smallest amount of energy to be used as is required.  This saves from heating up one big boiler or turning on a huge burner to do the job a small unit can handle.

System longevity - A staged system outlives a single system because it works half as much.  Much of the cost in today's modern heating systems is in the installation.  This makes a great job last longer. 

 Added Punch -  The combined output is generally greater than required when sizing a single boiler system and gives the homeowner expandability and quicker recovery times for hot water.