SQL Server - Server name same as the instance name and most common use for instances?
Servers are like the physical location. Instances are the actual SQL Server functionality. They are what you connect to in order to use SQL Server.


Each Instance belongs to only one Server. 
You can have multiple instances per Server (so T_COMPUTER\SQLEXPRESS and T_COMPUTER\SQLSERVER could be 2 instances on the same Server).
An Instance is a fully self contained SQL Server installation.
You can have a default instance configured, which is the Instance accessed when you connect to the Server directly (i.e. T_COMPUTER, actually connects to T_COMPUTER\INSTANCE)