Why is it not advisable to have the database and web server on the same machine?

Question

Listening to Scott Hanselman\'s interview with the Stack Overflow team (part 1 and 2), he was adamant that the SQL server and application server should be on separate machin

Answer 1

It doesn't really matter (you can quite happily run your site with web/database on the same machine), it's just the easiest step in scaling..

It's exactly what StackOverflow did - starting with single machine running IIS/SQL Server, then when it started getting heavily loaded, a second server was bought and the SQL server was moved onto that.

If performance is not an issue, do not waste money buying/maintaining two servers.