In some cases you might have to stop a SQL Server instance. For example: you want to install SQL Server 2012, but you have a previous version like SQL Server 2008 installed. and, both of two versions are set the default TCP/IP port number, which is 1433 for SQL Server. so in this case you will have to stop the old SQL Server and install new one.
Actually we want to stop entire old SQL Server system for the new installation, but there is no such a way to stop entire SQL Server system but you will have to uninstall the old one, so you can just stop a SQL Server instance which you have created in the previous SQL Server edition.