Being fairly unobtrusive, I find modelines convenient for making explicit the proper indenting mode for source files. This is useful in distributed development, where the project doesn't specify a preferred code style. Even if other developers don't use vim, they should be able to understand how they are expected to indent their PR.

Moreover, they're super handy for loading the proper syntax highlighting when working with template engines (e.g. Jinja2) to produce anything other than HTML.