If you have SQL Server Reporting Services in SharePoint 2007 Native Mode, this would have required SharePoint installed on the SSRS server. This is probably why your set up looks like it does - it's much cheaper to install a non-accessible SharePoint instance on your DB server than it is to install a DB instance on one of your WFE machines.
It's just a result of licensing, costs, etc. I'm not entirely sure what the required set up is for using SSRS in SharePoint 2010 so I don't know if this same configuration is required.
*Note that SSRS can really drag down your farm performance if you are using only 1 SQL Server to host the SharePoint Content Databases AND your SSRS instance. It's better to set up a server just to handle the SSRS stuff and leave your other server(s) to handle just SharePoint related tasks. Just a random aside based on observations.
This blog post discusses the server configuration for Report Services and SharePoint. Look at 03 - Available Server.