sys.database_files / sys.filegroups VS sys.sysfiles / sys.sysfilegroups
Almost always, backward compatibility. Often for end user compatibility but also for internal purposes.

I'd swear I was reading an article earlier this week that mentioned one of the old views, possibly sysfiles, having been 99% removed during SQL2005 development but the final 1% proved to difficult/risky to eliminate.

As a general rule, target the most recent versions of any system tables/views in your scripts, they're more likely to still be there in subsequent releases.