Web development is all about breaking up your application into pieces. Multiple pages make up a site. UserControls are used to break up pages into smaller pieces, and custom libraries are often used for either framework value, or as GUI libraries. Any time you start working with these scenarios, assemblies and references are going to start to enter the equation.
In a perfect world, assembly references would be easy, and in many cases they are. But what happens when you need to support more complex scenarios? I happen to live and breathe components, working for a UI tools company, so I run into these questions every day.
Yesterday, I had a customer who needed to get an update for a bugfix, but wanted to limit the exposure (and regression testing) by applying the fix to a single page (or component) rather than the entire site. I happen to think this is a great idea and can dramatically reduce 'risk' when updating software.