I think one big advantage could be seemless integration of all applications. Genera on Lisp Machines [1] and McClim [2] illustrate how it works.
[1] https://news.ycombinator.com/item?id=1878220
[2] https://common-lisp.net/project/mcclim/excite.html
In classical OS (Linux, Windows, Mac) the developer is responsible for seemless integration (in KDE or MS Office for instance). It is easy to break design guidelines here which causes ugly interoperation, or even lack of interoperation.