Nostalgia. Software archeology. Language hobbyists wishing to keep it in a working state.
Besides that there is little reason to use Dylan, nowadays.
(But see BruceM's comment about using this Dylan as a foundation for further development.)
It was once developed by Apple to replace Object Pascal and C++ for application and also systems development. But with NeXT bought by Apple that was the role of Objective C and many years later now also for Swift.
This particular version of Dylan was developed by Harlequin - the product name was DylanWorks. Harlequin was bought by Globalgraphics. Globalgraphics used the Postscript raster image processor developed by Harlequin and spun off the LispWorks system into its own company. Years later DylanWorks and even later MLWorks were open sourced. One idea of Harlequin was to develop a range of modern programming languages, implementations for those and development environments: Postscript, Common Lisp, Dylan, ML, ... The implementations were closed source and proprietary. When Apple announced their Dylan, Harlequin hoped to follow a market leader by providing an implementation for platforms Apple would not cover: Unix, Windows, ... Also Harlequin might have hoped to use their Dylan themselves for development of software applications.
But Harlequin in this forms does not exist anymore, DylanWorks has been open sourced and Apple has long ago lost interest in Dylan.
On the positive side: this Dylan is free and open source and interested persons can study and, maybe, use it.
Lisp users might be interested to study the prototyped implementation of Dylan for LispWorks. This implementation was not their shipped product, though.
https://github.com/dylan-lang/opendylan/tree/5f9225d246d771c...
That particular version is another example of a non-trivial programming language built in Lisp - especially given that it also has a non Lisp-like syntax.