为什么在SQL Server 2012管理Studio中“关闭现有的目标数据库连接”?

I am normally using SQL Server 2012 Management Studio to restore a SQL Server database from a "bak" file. I do this by overwriting an existing database. In the "Options" page, there is a check-box labeled "Close existing connections to destination database", which I mostly check because the target database is always "in use", even if I just restarted the MS SQL Service and I am sure that there are no applications using it. I am normally using SQL Server 2012 Management