I undersand what you're going through. While my fellow DBA and I have admin access to all the servers we need, it is not unusual for a network engineer in our organization to install SQL Server somewhere without our knowledge and we learn about it only when there's a problem, leaving it up to us to reconfigure the instance the way it should have been initially.

Here's my non-technical explanation:

When installing SQL Server there are many options that can be selected and decisions that can be made. If one just clicks Next all the way through accepting all the defaults the applications will probably work initially, but it is likely not installed in such a way that offers the best performance, flexibity, security and reliability. Some non-optimum configuration options are difficult to correct down the road when users are depending on the databases to perform their jobs.

If you want database applications to function as quickly, reliabley and securely as possible with a miniumum amount of downtime if something does go wrong, a person with DBA knowlege and experience should perform the installation.