It's common in javascript at least because you're delivering the code across the network, frequently, and at dev time, you're probably pulling in large libraries of components and frameworks that you may not use in their entirety. Since the load time is one of the core metrics used by SEO, and has shown to have a significant impact on conversion rates, it's worth the investment to remove code that's not being used.

I'm sure there are other places where it's applicable, but modern javascript and webapps have to be the quintessential use case.