There are plenty of reasons why corporations want to have a particular piece of software running over another, we won't go into all of that here. I had worked with a similar product, mSQL, several years ago on Unix systems and had always been on the lookout for a low-cost, efficient database engine for Windows systems. Currently both mSQL and MySQL are both available for Windows. In comparing the two systems, I chose to work with the current version of MySQL as the ODBC drivers for MySQL are much more refined. Secondly, with several Windows based administration tools that work well with MySQL, the maintenance burden has been made much lighter.
Every single page has at least one database call, and many have two or three database calls.
Converting the site to MySQL done to improve the speed of the responses. With the entire site being finished, I wanted to make the transition to MySQL as painless as possible. But simply changing the ODBC configuration from pointing to the Access file to pointing to the MySQL database, transitioning was extremely simple. The only changes to the SQL queries had to change the format of date related fields. This is a process you would have to do if you were switching from Access to Microsoft's SQL Server.