[sql-server] What is the point of "Initial Catalog" in a SQL Server connection string?

Every SQL Server connection string I ever see looks something like this:

Data Source=MyLocalSqlServerInstance;Initial Catalog=My Nifty Database;
    Integrated Security=SSPI;

Do I need the Initial Catalog setting? (Apparently not, since the app I'm working on appears to work without it.)

Well, then, what's it for?

Setting an Initial Catalog allows you to set the database that queries run on that connection will use by default. If you do not set this for a connection to a server in which multiple databases are present, in many cases you will be required to have a USE statement in every query in order to explicitly declare which database you are trying to run the query on. The Initial Catalog setting is a good way of explicitly declaring a default database.

This is the initial database of the data source when you connect.

Edited for clarity:

If you have multiple databases in your SQL Server instance and you don't want to use the default database, you need some way to specify which one you are going to use.